Disable test_PlaceholderOnTopStackSwitch() on MacOS I-builds

PartOnTopManagerTest.test_PlaceholderOnTopStackSwitch() fails stabily in MacOS I-builds.

Since its unclear what is wrong, the test is now disabled to reduce noise in test results.

See: #3893

Author: trancexpress

tests/org.eclipse.e4.ui.tests/META-INF/MANIFEST.MF

@@ -39,6 +39,7 @@ Import-Package: jakarta.annotation,
  jakarta.inject,
  org.osgi.service.event,
  org.junit.jupiter.api;version="[5.14.0,6.0.0)",
+ org.junit.jupiter.api.condition;version="[5.14.0,6.0.0)",
  org.junit.jupiter.api.extension;version="[5.14.0,6.0.0)",
  org.junit.platform.suite.api;version="[1.14.0,2.0.0)",
  org.opentest4j

tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/workbench/PartOnTopManagerTest.java

@@ -19,6 +19,7 @@
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import jakarta.inject.Inject;
+import org.eclipse.core.runtime.Platform;
 import org.eclipse.e4.ui.internal.workbench.PartOnTopManager;
 import org.eclipse.e4.ui.model.application.MApplication;
 import org.eclipse.e4.ui.model.application.ui.MContext;
@@ -33,6 +34,7 @@
 import org.eclipse.e4.ui.workbench.IWorkbench;
 import org.eclipse.e4.ui.workbench.modeling.EModelService;
 import org.eclipse.e4.ui.workbench.modeling.EPartService;
+import org.junit.jupiter.api.Assumptions;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
 
@@ -136,6 +138,8 @@ public void test_PartOnTopStackSwitch() {
 
 	@Test
 	public void test_PlaceholderOnTopStackSwitch() {
+		Assumptions.assumeFalse(Platform.OS.isMac() && System.getenv("ANT_HOME") != null, // 'ANT_HOME' is defined during I-builds, but not CI jobs on PRs
+				"test fails stably on MacOS I-builds, its unclear how to fix it, see: https://github.com/eclipse-platform/eclipse.platform.ui/issues/3893");
 		MWindow window = ems.createModelElement(MWindow.class);
 		application.getChildren().add(window);
 		application.setSelectedElement(window);