Skip forceActive in generic editor tests when shell is already active

UITestUtil.forceActive() has a documented one second floor, made up of
four waitForJobs() calls, see bug 417258#c27.
AbstratGenericEditorTest.setUp() called it on every test method, so all
45 tests in the bundle paid it.

Measured on a full CI run, org.eclipse.ui.genericeditor.tests takes
66.3s, and 58.5s of that (88%) is a flat per-test floor of about 1.25s
that is present in every one of its 14 test classes.

Only force the shell active when it is not already the active shell,
which is the same condition forceActive() itself returns.

Author: vogella

tests/org.eclipse.ui.genericeditor.tests/src/org/eclipse/ui/genericeditor/tests/AbstratGenericEditorTest.java

@@ -30,6 +30,8 @@
 import org.eclipse.core.resources.IResource;
 import org.eclipse.core.resources.ResourcesPlugin;
 
+import org.eclipse.swt.widgets.Shell;
+
 import org.eclipse.ui.IEditorInput;
 import org.eclipse.ui.IWorkbenchWindow;
 import org.eclipse.ui.PlatformUI;
@@ -62,7 +64,11 @@ public void setUp() throws Exception {
 		project.setDefaultCharset(StandardCharsets.UTF_8.name(), null);
 		waitForJobs(100, 5000);
 		window= PlatformUI.getWorkbench().getActiveWorkbenchWindow();
-		forceActive(window.getShell());
+		Shell shell= window.getShell();
+		// forceActive() has a one second floor, so skip it when the shell is already active
+		if (shell.getDisplay().getActiveShell() != shell) {
+			forceActive(shell);
+		}
 		createAndOpenFile();
 	 }