Mocks created by JUnit4 tests are not cleaned up when run with JUnit5

[tsmock]

Please provide the following information:

• Version of JMockit that was used: 1.49.a (see https://github.com/JOSM/jmockit1/tree/josm-fixes )

• Description of the problem or enhancement request:
  When running JUnit4 and JUnit5 tests together, JMockit does not properly cleanup after mocks are created in the JUnit4 tests.

Problem code:
https://github.com/jmockit/jmockit1/blob/master/main/src/mockit/internal/startup/JMockitInitialization.java#L34

Workaround for non-patched releases:
Create a new mockit.internal.state.SavePoint before each test (i.e., in a @Before method) and then call the rollback method in an @After method. See https://josm.openstreetmap.de/changeset/18551/josm/ for an example using a custom TestRule.

• Check the following:

• If a defect or unexpected result, JMockit project members should be able to reproduce it.
  For that, include an example test (perhaps accompanied by a Maven/Gradle build script) which
  can be executed without changes and reproduces the failure.

• If an enhancement or new feature request, it should be justified by an example test
  demonstrating the validity and usefulness of the desired enhancement or new feature.

• The issue does not fall outside the scope of the project (for example, attempting to use
  JMockit APIs from Groovy or Scala code, or with an Android runtime).

• The JDK where the problem occurs is a final release, not a development build.