Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.
The test suite includes a few basic tests and a very crude regression
test, which just concatenates the to_string() of all tokens and checks
the String's hash to be equal. This relies on the format of
HTMLToken::to_string() to stay the same, which is not ideal.