Build testcase_lowdpi tests as application bundles on macOS

As stand-alone executables are not tracked by Launch Services,
which is responsible for applying the low-dpi override at runtime.

Change-Id: Ifac71dfa9b9ffa5ab1302dfca8fa1f5ddd491082
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

mkspecs/features/testcase.prf

@@ -19,6 +19,10 @@ testcase_lowdpi {
             error("QMAKE_INFO_PLIST already set, can't apply testcase_lowdpi")
 
         QMAKE_INFO_PLIST = $$QMAKESPEC/Info.plist.disable_highdpi
+        # Needs to be bundle so that Launch Services will pick
+        # up and apply the NSHighResolutionCapable=NO state.
+        CONFIG += app_bundle
+
     } else {
         # TODO: Add support for other platforms if possible
     }