Fix race condition in MMenuItemTest by adding event loop processing

Fixes #1737

The test testElementHierarchyInContext_HandledItem was experiencing
intermittent failures due to a race condition. After creating handler
processing addons and activating parts, the handler registration may
not have completed before the test triggered the menu item selection.

This commit adds calls to contextRule.spinEventLoop() in both
testElementHierarchyInContext_DirectItem and
testElementHierarchyInContext_HandledItem to ensure all pending UI
events are processed before triggering menu item selection. This is
a standard pattern for preventing race conditions in Eclipse UI tests.

The spinEventLoop() method processes all pending events in the Display
event queue, ensuring that handler registration and context updates
have completed before the test proceeds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: vogella

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

@@ -763,6 +763,9 @@ public void execute(MUIElement uiElement, MMenuItem menuItem,
 		activePart.getContext().set("key", "active");
 		inactivePart.getContext().set("key", "inactive");
 
+		// Ensure all pending UI events are processed before triggering the menu item
+		contextRule.spinEventLoop();
+
 		assertFalse(executed[0]);
 
 		Object widget1 = menuItem.getWidget();
@@ -836,6 +839,10 @@ public void execute(MUIElement uiElement, MMenuItem menuItem,
 		activePart.getContext().set("key", "active");
 		inactivePart.getContext().set("key", "inactive");
 
+		// Ensure all pending UI events are processed before triggering the menu item
+		// This prevents race conditions where the handler may not be fully registered yet
+		contextRule.spinEventLoop();
+
 		assertFalse(executed[0]);
 		assertEquals(activePart, window.getContext().get(EPartService.class)
 				.getActivePart());