Allow tests to wait on reconciler threads to finish

[trancexpress]

While looking into various JDT UI test fails, e.g. eclipse-jdt/eclipse.jdt.ui#2287, I noticed that the we have no option to wait on the thread started here: org.eclipse.jface.text.reconciler.AbstractReconciler.BackgroundWorker

E.g. I can get the addition of the project info at this stack trace:

"org.eclipse.jdt.internal.ui.text.JavaReconciler" #86 [165284] daemon prio=1 os_prio=0 cpu=0.86ms elapsed=66.00s tid=0x00007fe978058760 nid=165284 at breakpoint [0x00007fe9037f9000]
   java.lang.Thread.State: RUNNABLE
        at org.eclipse.jdt.internal.core.JavaElement.openWhenClosed(JavaElement.java:585)
        at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:284)
        at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:270)
        at org.eclipse.jdt.internal.core.JavaProject.getJavaProjectElementInfo(JavaProject.java:2088)
        at org.eclipse.jdt.internal.core.JavaProject.newNameLookup(JavaProject.java:2823)
        at org.eclipse.jdt.internal.core.SearchableEnvironment.<init>(SearchableEnvironment.java:117)
        at org.eclipse.jdt.internal.core.SearchableEnvironment.<init>(SearchableEnvironment.java:158)
        at org.eclipse.jdt.internal.core.CancelableNameEnvironment.<init>(CancelableNameEnvironment.java:32)
        at org.eclipse.jdt.internal.core.CompilationUnitProblemFinder.process(CompilationUnitProblemFinder.java:276)
        at org.eclipse.jdt.internal.core.CompilationUnitProblemFinder.process(CompilationUnitProblemFinder.java:375)
        at org.eclipse.jdt.internal.core.ReconcileWorkingCopyOperation.makeConsistent(ReconcileWorkingCopyOperation.java:244)
        at org.eclipse.jdt.internal.core.ReconcileWorkingCopyOperation.executeOperation(ReconcileWorkingCopyOperation.java:108)
        at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:751)
        at org.eclipse.jdt.internal.core.JavaModelOperation.runOperation(JavaModelOperation.java:816)
        at org.eclipse.jdt.internal.core.CompilationUnit.reconcile(CompilationUnit.java:1431)
        at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:132)
        at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy$1.run(JavaReconcilingStrategy.java:94)
        at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
        at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:91)
        at org.eclipse.jdt.internal.ui.text.java.JavaReconcilingStrategy.reconcile(JavaReconcilingStrategy.java:158)
        at org.eclipse.jdt.internal.ui.text.CompositeReconcilingStrategy.reconcile(CompositeReconcilingStrategy.java:94)
        at org.eclipse.jdt.internal.ui.text.JavaCompositeReconcilingStrategy.reconcile(JavaCompositeReconcilingStrategy.java:107)
        at org.eclipse.jface.text.reconciler.MonoReconciler.process(MonoReconciler.java:77)
        at org.eclipse.jface.text.reconciler.AbstractReconciler$BackgroundWorker.run(AbstractReconciler.java:205)
        at java.lang.Thread.runWith(java.base@21.0.10/Thread.java:1596)
        at java.lang.Thread.run(java.base@21.0.10/Thread.java:1583)

While the entire org.eclipse.jdt.ui.tests.core.ImportOrganizeTest finishes.

We should have some option to be able to wait on this thread to exit, to ensure tests don't run with parallel tasks from previous tests.

[trancexpress]

Lets first see if this helps: eclipse-jdt/eclipse.jdt.ui#2897

If it does, we can think about adding a better type of waiting.