make use of top-level return() to beautify code

Change-Id: Ia192aab9981a790f1a74a83d4894395b454ab0c1
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

mkspecs/features/testcase.prf

@@ -1,4 +1,4 @@
-have_target {
+!have_target: return()
 
 # qt_build_config.prf disables execptions for all Qt modules which don't
 # explicitly turn it on again, while the qmake default is to build with
@@ -204,5 +204,3 @@ macx-xcode:bundle:isEmpty(QMAKE_BUNDLE_EXTENSION) {
     QMAKE_BUNDLE_EXTENSION = .xctest
 }
 
-} # have_target
-