False positive, java/unused-reference-type and JUnit 5 classes

JUnit Tests using JUnit 5 are reported as java/unused-reference-type. This is because the JUnit 5 convention is to use package scoped class and method names. This rule should be checking for methods with JUnit annotations (e.g. `@Test`, `@BeforeEach`, `@ParameterizedTest`). The [current check](https://github.com/github/codeql/blob/3059ce307009d40afb690aa712c96e422cf76753/java/ql/src/Violations%20of%20Best%20Practice/Dead%20Code/DeadRefTypes.ql#L44:L45) only looks for subsclassing of TestCase, which hasn't been applicable since JUnit 4.

**Code samples or links to source code**

Example test code that would be reported as unused.
```
class MyTest
{
@Test
void test()
{
// some test code.
}
}
```
Example false positive.
https://github.com/real-logic/aeron/security/code-scanning/1583

Read java/ql/src/Violations of Best Practice/Dead Code/DeadRefTypes.ql around the current TestCase check, then inspect the query's existing tests and run the relevant CodeQL test suite. Done means JUnit 5 classes with annotations such as @Test, @BeforeEach, or @ParameterizedTest are no longer reported as unused reference types, while the existing detection still works.

Written by the indexing model from the issue text.