Clang: Tests: Fix flaky testCompleteProjectDependingCodeAfterChangingProject

After changing the project, ensure that the project part is updated and
thus the backend is notified about it, otherwise we run into a race
condition.

Change-Id: Iea1ff16156350d10982cfd7fac3e8e5220eb4726
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangcodemodel/test/clangcodecompletion_test.cpp

@@ -655,6 +655,7 @@ void ClangCodeCompletionTest::testCompleteProjectDependingCodeAfterChangingProje
 
         // Check completion with project configuration 2
         QVERIFY(projectLoader.updateProject({{"PROJECT_CONFIGURATION_2"}}));
+        openEditor.waitUntilBackendIsNotified();
         proposal = completionResults(openEditor.editor());
 
         QVERIFY(!hasItem(proposal, "projectConfiguration1"));