Flaky UI tests

[LorenzoBettini]

Under investigation

Typically, on GitHub Actions macOS arm there are always a few flaky tests (the test succeeds because we have a Maven property to rerun flaky tests).

Some examples:

Flakes: 
org.eclipse.xtext.builder.impl.DisabledBuilderTest.testDisabledProject
Run 1: DisabledBuilderTest.testDisabledProject:51
Run 2: PASS

Flakes: 
org.eclipse.xtend.ide.tests.builder.SaveWithReconciliationQueuedBuildDataTest.addPublicMethod
Run 1: SaveWithReconciliationQueuedBuildDataTest>AbstractSingleEditorQueuedBuildTest.addPublicMethod:1003->AbstractQueuedBuildDataTest.assertThereAreDeltas:91->AbstractQueuedBuildDataTest.assertDeltas:209->AbstractQueuedBuildDataTest.confirmDeltas:227->Assert.assertTrue:42->Assert.fail:89 There are unconfirmed changes: [mypackage.Bar]
Run 2: PASS

org.eclipse.xtend.ide.tests.compiler.ResolvingCrossReferenceDuringIndexingTest.testResolvingXFunctionTypeRef_3
Run 1: ResolvingCrossReferenceDuringIndexingTest.testResolvingXFunctionTypeRef_3:283->testResolvingXFunctionTypeRef:421->assertNoErrorsInWorkspace:460->Assert.assertFalse:65->Assert.assertTrue:42->Assert.fail:89 org.junit.Assert cannot be resolved to a type.
Run 2: PASS

org.eclipse.xtend.ide.tests.macros.JdtBasedProcessorProviderTest.testClassLoaderSeesAllUpstreamProjects_01
Run 1: JdtBasedProcessorProviderTest.testClassLoaderSeesAllUpstreamProjects_01:225 Workspace contained errors: 
- A.xtend:1 - Couldn't find the mandatory library 'org.eclipse.xtext.xbase.lib' 2.8.0 or higher on the project's classpath.(org.eclipse.xtend.ide.xtend.check.fast)
- A.xtend:3 - annotation.MyAA cannot be resolved to an annotation type.(org.eclipse.xtend.ide.xtend.check.fast)
Run 2: PASS