[Test] Several tests failing since (at least) I20250620-1800

1482 failures for Mac: https://download.eclipse.org/eclipse/downloads/drops4/I20250620-1800/testresults/html/org.eclipse.jdt.ui.tests_ep437I-unit-macosx-x86_64-java21_macosx.cocoa.x86_64_21.html

1481 for **Linux** 3 days later: https://download.eclipse.org/eclipse/downloads/drops4/I20250623-1800/testresults/html/org.eclipse.jdt.ui.tests_ep437I-unit-linux-x86_64-java24_linux.gtk.x86_64_24.html

Most (if not all) of the failures show something like this:

```java
Multiple Failures (2 failures) org.eclipse.core.internal.resources.ResourceException: Resource '/TestSetupProject' does not exist. org.eclipse.jdt.core.JavaModelException: TestSetupProject does not exist

org.opentest4j.MultipleFailuresError: Multiple Failures (2 failures)
org.eclipse.core.internal.resources.ResourceException: Resource '/TestSetupProject' does not exist.
org.eclipse.jdt.core.JavaModelException: TestSetupProject does not exist
Suppressed: org.eclipse.core.internal.resources.ResourceException: Resource '/TestSetupProject' does not exist.
at org.eclipse.core.internal.resources.Resource.checkExists(Resource.java:364)
at org.eclipse.core.internal.resources.Resource.checkAccessible(Resource.java:238)
at org.eclipse.core.internal.resources.Project.checkAccessible(Project.java:183)
at org.eclipse.core.internal.resources.Folder.assertCreateRequirements(Folder.java:43)
at org.eclipse.core.internal.resources.Folder.create(Folder.java:101)
at org.eclipse.core.internal.resources.Folder.create(Folder.java:129)
at org.eclipse.jdt.internal.ui.util.ResourcesUtility.createFolder(ResourcesUtility.java:77)
at org.eclipse.jdt.internal.ui.util.CoreUtility.createFolder(CoreUtility.java:64)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:754)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:732)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:718)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:705)
at org.eclipse.jdt.testplugin.JavaProjectHelper.addSourceContainer(JavaProjectHelper.java:693)
at org.eclipse.jdt.ui.tests.quickfix.CleanUpTestCase.setUp(CleanUpTestCase.java:115)
at org.eclipse.jdt.ui.tests.quickfix.SaveParticipantTest.setUp(SaveParticipantTest.java:74)
at java.base/java.lang.reflect.Method.invoke(Method.java:565)
Caused by: java.lang.Exception: Resource '/TestSetupProject' does not exist.
at org.eclipse.core.internal.resources.ResourceException.provideStackTrace(ResourceException.java:42)
at org.eclipse.core.internal.resources.ResourceException.(ResourceException.java:38)
... 16 more
Suppressed: Java Model Exception: Error in Java Model (code 969): TestSetupProject does not exist
at org.eclipse.jdt.internal.core.JavaElement.newNotPresentException(JavaElement.java:548)
at org.eclipse.jdt.internal.core.JavaModelManager.getPerProjectInfoCheckExistence(JavaModelManager.java:2549)
at org.eclipse.jdt.internal.core.JavaProject.getPerProjectInfo(JavaProject.java:2361)
at org.eclipse.jdt.internal.core.JavaProject.getOutputLocation(JavaProject.java:2176)
at org.eclipse.jdt.internal.core.JavaProject.setRawClasspath(JavaProject.java:3544)
at org.eclipse.jdt.testplugin.JavaProjectHelper.lambda$1(JavaProjectHelper.java:626)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2457)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2477)
at org.eclipse.jdt.testplugin.JavaProjectHelper.clear(JavaProjectHelper.java:634)
at org.eclipse.jdt.ui.tests.quickfix.CleanUpTestCase.tearDown(CleanUpTestCase.java:127)
at java.base/java.lang.reflect.Method.invoke(Method.java:565)
```

Since the same amount of test ran for I20250621-1800 and there is only [1 failure](https://download.eclipse.org/eclipse/downloads/drops4/I20250621-1800/testresults/html/org.eclipse.jdt.ui.tests_ep437I-unit-macosx-aarch64-java21_macosx.cocoa.aarch64_21.html), I assume it's an infrastructure problem.

I didn't investigate further FTM. Comments are welcome.

Start with the failing test setup and cleanup paths in CleanUpTestCase.setUp/tearDown and SaveParticipantTest.setUp, then inspect JavaProjectHelper.addSourceContainer and clear. Compare the linked macOS and Linux test reports with the passing I20250621-1800 run. Done means identifying the infrastructure or regression cause and restoring the affected JDT UI tests.

Written by the indexing model from the issue text.