Add unit tests for TUnit

tests/test_proof/bad/enums_unit-empty.catala_en

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+## Article
+
+```catala
+declaration enumeration E:
+  -- Case1 content integer
+  -- Case2
+
+declaration scope A:
+  context x content E
+  context y content integer
+
+scope A:
+  definition x equals Case1 content 2
+  definition y under condition match x with pattern
+    -- Case1 of i : i > 0
+    -- Case2 : false consequence equals 2
+  definition y under condition match x with pattern
+    -- Case1 of i : false
+    -- Case2 : true consequence equals 2
+```

tests/test_proof/bad/enums_unit-overlap.catala_en

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+## Article
+
+```catala
+declaration enumeration E:
+  -- Case1 content integer
+  -- Case2
+
+declaration scope A:
+  context x content E
+  context y content integer
+
+scope A:
+  definition x equals Case1 content 2
+  definition y under condition match x with pattern
+    -- Case1 of i : true
+    -- Case2 : true consequence equals 2
+  definition y under condition match x with pattern
+    -- Case1 of i : false
+    -- Case2 : true consequence equals 2
+```

tests/test_proof/bad/output/enums_unit-empty.catala_en.Proof

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+[ERROR] [A.y] This variable might return an empty error:
+  --> tests/test_proof/bad/enums_unit-empty.catala_en
+   | 
+10 |   context y content integer
+   |           ^
+   + Article
+Counterexample generation is disabled so none was generated.
+[RESULT] [A.y] No two exceptions to ever overlap for this variable
+[RESULT] [A.x] This variable never returns an empty error
+[RESULT] [A.x] No two exceptions to ever overlap for this variable

tests/test_proof/bad/output/enums_unit-overlap.catala_en.Proof

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+[RESULT] [A.y] This variable never returns an empty error
+[ERROR] [A.y] At least two exceptions overlap for this variable:
+  --> tests/test_proof/bad/enums_unit-overlap.catala_en
+   | 
+10 |   context y content integer
+   |           ^
+   + Article
+Counterexample generation is disabled so none was generated.
+[RESULT] [A.x] This variable never returns an empty error
+[RESULT] [A.x] No two exceptions to ever overlap for this variable

tests/test_proof/good/enums_unit.catala_en

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+## Article
+
+```catala
+declaration enumeration E:
+  -- Case1 content integer
+  -- Case2
+
+declaration scope A:
+  context x content E
+  context y content integer
+
+scope A:
+  definition x equals Case1 content 2
+  definition y under condition match x with pattern
+    -- Case1 of i : true
+    -- Case2 : false consequence equals 2
+  definition y under condition match x with pattern
+    -- Case1 of i : false
+    -- Case2 : true consequence equals 2
+```

tests/test_proof/good/output/enums_unit.catala_en.Proof

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+[RESULT] [A.y] This variable never returns an empty error
+[RESULT] [A.y] No two exceptions to ever overlap for this variable
+[RESULT] [A.x] This variable never returns an empty error
+[RESULT] [A.x] No two exceptions to ever overlap for this variable