CMake: Add "test" target to cmake builds

Always show a test target for cmake projects. This will show even
if there are no tests in the project. But I think it is more annoying
*not* to be able to run tests via the locator when tests are available
than getting an error message when attempting that with a project that
does not have tests.

Task-number: QTCREATORBUG-18323
Change-Id: Iba85aa868cb9bfe6c3f44a7ffff620d081d3082f
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.cpp

@@ -404,9 +404,14 @@ QString CMakeBuildStep::installTarget()
     return QString("install");
 }
 
+QString CMakeBuildStep::testTarget()
+{
+    return QString("test");
+}
+
 QStringList CMakeBuildStep::specialTargets()
 {
-    return { allTarget(), cleanTarget(), installTarget() };
+    return { allTarget(), cleanTarget(), installTarget(), testTarget() };
 }
 
 //