Add TestMethodsShouldBeVoid recipe to fix tests not discovered

[timtebeek]

Summary

• Implements recipe to fix test methods with non-void return types
  • fixes Recipe to discover test methods with a non-void return type #790
• Changes return type to void for test methods annotated with @Test, @ParameterizedTest, @RepeatedTest, etc.
• Removes return statements from method bodies while preserving nested lambda/class returns

Details

Some test methods with non-void return types were not being discovered or run correctly, causing broken test coverage and warnings in JUnit 5.13+. This recipe automatically fixes test method signatures to ensure proper test discovery and execution.

Changes made:

1. Created TestMethodsShouldBeVoid recipe that:

   • Detects test methods with non-void return types
   • Changes the return type to void
   • Removes return statements from the method body
   • Preserves return statements in nested lambdas and anonymous classes

2. Added comprehensive test coverage with 10 test cases covering:

   • Simple return type changes
   • Return statement removal
   • Preservation of returns in lambdas
   • Preservation of returns in anonymous classes
   • Parameterized tests
   • Repeated tests
   • Multiple return statements
   • Method invocation preservation

Example transformation:

// Before
@Test
int testMethod() {
    int i = 42;
    return i;
}

// After
@Test
void testMethod() {
    int i = 42;
}

Test plan

• All 10 unit tests pass
• Recipe correctly handles various test annotations
• Return statements in nested contexts are preserved
• Method invocations are preserved when removing returns

Fixes #790

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