Re-enable Display.post tests

These tests have been disabled overtime because making sure they
pass consistently on Jenkins is difficult (See
[407862](https://bugs.eclipse.org/bugs/show_bug.cgi?id=407862) and
[553754](https://bugs.eclipse.org/bugs/show_bug.cgi?id=553754))

With GitHub actions these problems seem to be less of an issue, so we
should enable these tests again, at least on GHA.

This first commit is a test to see if they pass on GHA consistently, if
so adding a new check so that tests can be skipped on Jenkins releng
infra can be added.

Part of #2571

Author: jonahgraham

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_widgets_Display.java

@@ -1065,14 +1065,6 @@ public void test_mapLorg_eclipse_swt_widgets_ControlLorg_eclipse_swt_widgets_Con
 
 @Test
 public void test_postLorg_eclipse_swt_widgets_Event() {
-	if (SwtTestUtil.isGTK || SwtTestUtil.isCocoa || SwtTestUtil.isWindows ) {
-		//TODO Fix/revisit GTK, Cocoa and Win10 failure test-case via bug 553754
-		if (SwtTestUtil.verbose) {
-			System.out.println("Excluded test_postLorg_eclipse_swt_widgets_Event(org.eclipse.swt.tests.junit.Test_org_eclipse_swt_widgets_Display)");
-		}
-		return;
-	}
-
 	final int KEYCODE = SWT.SHIFT;
 
 	Display display = new Display();