Remove now-invalid @Override when migrating TestCase subclasses

[timtebeek]

• Fixes JUnit 4 to 5 migration leaves @Override on methods that overrode TestCase, breaking compilation #1103

MigrateJUnitTestCase only removed @Override as a side effect of adding a Jupiter lifecycle annotation. When a TestCase subclass had already been half-migrated to JUnit 4 — setUp() carrying both @Override and @Before — that branch was skipped, so extends TestCase was removed while the @Override stayed behind and the class no longer compiled. Same for getName() and countTestCases(), which never hit that branch at all.

Every method declaration in a migrated TestCase subclass is now checked against the supertypes it will still have after migration, skipping the junit.framework types that are going away. @Override is only removed when nothing else declares a matching method, so these keep it:

• toString(), equals(), hashCode() (declared on Object, and on TestCase only incidentally)
• implementations of an interface the class declares
• overrides of an intermediate base class that itself declares the method

RemoveAnnotationVisitor handles the removal so the leftover blank line is cleaned up; the body is detached first so @Override on methods of anonymous classes inside is untouched.

One existing expectation changed: in convertExtendedTestCase, MathTest.setUp() overrides CTest.setUp(), which CTest declares itself, so the @Override is now retained rather than stripped.