Improve assertions and avoid race condition in MarkerTest

[HeikoKlare]

• add further assertions for the number of affected resources
• make MarkersChangeListener.assertChange() wait for asynchronously processed change events to avoid race conditions

Addresses race condition seen as test failure testMarkerDeltasMoveFile(TestInfo) (AutomatedResourceTests AllResourcesTests MarkerTest) in this test run: https://github.com/eclipse-platform/eclipse.platform/pull/2396/checks?check_run_id=59405642125

[number of markers for resource /MyProject/folder/subFile.txt] 
expected: 2
 but was: 1
org.opentest4j.AssertionFailedError: 
[number of markers for resource /MyProject/folder/subFile.txt] 
expected: 2
 but was: 1
	at org.eclipse.core.tests.resources.MarkersChangeListener.assertChanges(MarkersChangeListener.java:54)
	at org.eclipse.core.tests.resources.MarkerTest.testMarkerDeltasMoveFile(MarkerTest.java:977)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)

[github-actions]

Test Results

 1 956 files  +1 304   1 956 suites  +1 304   1h 36m 37s ⏱️ + 57m 12s
 4 743 tests ±    0   4 719 ✅ +   31   24 💤  -  31  0 ❌ ±0 
14 229 runs  +9 486  14 047 ✅ +9 359  182 💤 +127  0 ❌ ±0 

Results for commit 211ed2c. ± Comparison against base commit 690721b.