Prevent multiple execution of junit.runtime.tests and always clear workspace

[HannesWell]

Fixes #1360

[github-actions]

Test Results

   285 files  ±    0     285 suites  ±0   51m 49s ⏱️ + 5m 58s
 3 580 tests  -     1   3 504 ✅ +    1   76 💤 ± 0  0 ❌  - 1 
10 932 runs  +2 601  10 701 ✅ +2 579  231 💤 +24  0 ❌  - 1 

Results for commit 2ec27c4. ± Comparison against base commit 9a216f5.

This pull request removes 1 test.

org.eclipse.pde.junit.runtime.tests.JUnit5SuiteExecutionTest ‑ Unknown test

♻️ This comment has been updated with latest results.

[HannesWell]

Looks like JUnit5SuiteExecutionTest also called JUnitExecutionTest.setupProjects() but didn't have the CLEAR_WORKSPACE TestRule specified. Furthermore both the JUnit5SuiteExecutionTest and the JUnitExecutionTest were executed twice in verification builds because JUnitRuntimeTests suite defined in the package was not declared as testClass in the pom.xml (only referenced in the test.xml for I-Builds)

[HannesWell]

Great all tests pass again now.
As expected, the number of test execution has been reduced but only because the duplicated execution of JUnitExecutionTest and JUnit5SuiteExecutionTest is now avoided. I verified that both are now executed exactly once.