LibTest: Add a README documenting the high-level randomized approach

Author: https://github.com/Janiczek
Commit: https://github.com/SerenityOS/serenity/commit/32909d02b3
Pull-request: https://github.com/SerenityOS/serenity/pull/21191
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/timschumi

Userland/Libraries/LibTest/Randomized/README.md

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+# LibTest/Randomized
+
+Classes in this folder implement a randomized ("property based") testing library
+for SerenityOS.
+
+## Example
+
+```cpp
+RANDOMIZED_TEST_CASE(addition_commutative)
+{
+    GEN(int_a, Gen::unsigned_int());
+    GEN(int_b, Gen::unsigned_int());
+    CSSPixels a(int_a);
+    CSSPixels b(int_b);
+    EXPECT_EQ(a + b, b + a);
+}
+```
+
+The runner will then run the test case many times, with random data in each of
+the `GEN`'d variables. If it finds any set of values where the expectation fails,
+it shrinks it to a minimal failing example and presents it to the user:
+
+```
+Running test 'addition_commutative'.
+int_a = 0
+int_b = 1
+FAIL: /.../SomeTest.cpp:26: EXPECT_EQ(a + b, b + a) failed with lhs=0 and rhs=1
+Failed test 'addition_commutative' in 0ms
+```
+
+(Note that the runner most likely started out with much larger values like
+1587197 and 753361. The 0 and 1 are the shrunk, minimal values.)
+
+In the above case, the test runner tells us that `CSSPixels(0) + CSSPixels(1)`
+wasn't equal to `CSSPixels(1) + CSSPixels(0)`.
+
+> [!NOTE]
+> I made this failing example up. Don't worry, `CSSPixels` is fine :^)
+
+## Property based testing?
+
+Property based testing (PBT) involves generating random test inputs and checking
+that they satisfy certain properties. This allows testing a large number of
+possible cases automatically, and is exceptionally good at finding edge cases.
+
+When a failure is found, it's automatically shrunk to a minimal reproducing
+case. (This is good, because random values contain a lot of unimportant details
+that only hamper understanding of the root cause. See http://sscce.org)
+
+This particular implementation belongs to the "internal shrinking" family of PBT
+libraries (Hypothesis, minithesis, elm-test, ...). This is important for the DX
+as it gives the most resilient shrinkers without any user effort, compared to
+the "integrated shrinking" lazy-tree based implementations (Hedgehog etc.) or
+the manual shrinking ones (original QuickCheck etc.). (More information in this
+talk: https://www.youtube.com/watch?v=WE5bmt0zBxg)
+
+## Implementation
+
+On a very high level, the PBT runner remembers the random bits (`RandomRun`)
+used when generating the random value, and when it finds a value that fails the
+user's test, instead of trying to shrink the generated value it tries to shrink
+these random bits and then generate a new value from them. 
+
+This makes shrinking work automatically on any data you might want to generate.
+
+Generators are implemented in such a way that a smaller/shorter `RandomRun` will
+result in a simpler generated value.
+
+If the new RandomRun can be successfully turned into a value and if the test
+still fails on that new simpler value, we keep the RandomRun as our new best and
+try again. Once we can't improve things anymore, we turn on error reporting, run
+the test one last time with the final RandomRun, and report the failing value to
+the user.
+
+## Code organization
+
+- `TestResult.h`
+  - Defines an enum class TestResult.
+    This expands the typical "passed / failed": we also need to care about
+    a generator rejecting a RandomRun (eg. when the user calls the ASSUME(...)
+    macro with a predicate that can't be satisfied).
+
+- `Generator.h`
+  - Contains generators: fns of shape T(), eg. `u32 Gen::unsigned_int(u32 max)`
+    - These implicitly depend on a RandomnessSource held by the singleton
+      TestSuite.
+  - These can be called directly, but the top-level use by the user should always
+    happen via the GEN(...) macro which makes sure the generated value gets
+    logged to the user in case of a failure.
+  - Example:
+    `Gen::vector(1, 4, []() { return Gen::unsigned_int(5); })`
+    generates vectors of length between 1 and 4, of unsigned ints in range 0..5.
+    Eg. `{2,5,3}`, `{0}`, `{1,5,5,2}`.
+
+- `RandomnessSource.h`
+  - A source of random bits.
+  - There are two variants of `RandomnessSource`:
+    - Live: gives `AK/Random` u32 values and remembers them into a `RandomRun`
+    - Recorded: gives (replays) u32 values from a static `RandomRun`
+
+- `RandomRun.h`
+  - A finite sequence of random bits (in practice, `u32`s).
+  - Example: `{2,5,0,11,8,0,0,1}`
+
+- `ShrinkCommand.h`
+  - A high-level recipe for how to try and minimize a given `RandomRun`.
+  - For example, "zero this contiguous chunk of it" or "minimize the number on
+    this index using binary search".
+  - These later get interpreted by the PBT runner on a specific `RandomRun`.
+
+- `Chunk.h`
+  - A description of a contiguous `RandomRun` slice.
+  - Example: `Chunk{size = 4, index = 2}`:  [_,_,X,X,X,X,...]
+
+- `Shrink.h`
+  - Algorithms for interpreting `ShrinkCommand`s and the main shrinking loop
+
+- `TestCase.h`
+  - The `TestCase::randomized(...)` function contains the main testing loop