Append the environment's QT_PLUGIN_PATH to 'make check' targets

Without this, any test executable requiring a plugin path from
the environment's QT_PLUGIN_PATH will fail to run since the path is
overwritten when generating the 'make check' command, for example:

QT_PLUGIN_PATH=/path/to/qt/plugins \
LD_LIBRARY_PATH=/path/to/qt/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} \
./test_foo

A prepend config option is used for *PATH to preserve the envvar
value, so use the same option for QT_PLUGIN_PATH. The command above
then becomes:

QT_PLUGIN_PATH=/path/to/qt/plugins${QT_PLUGIN_PATH:+:$QT_PLUGIN_PATH} \
LD_LIBRARY_PATH=/path/to/qt/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH} \
./test_foo

Change-Id: I69b43327974915eae52f299fc4001effe93a491a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/features/qt_functions.prf

@@ -206,6 +206,7 @@ defineTest(qtAddTargetEnv) {
                 pluginpath.value += $$system_path($$qplug)
         }
         pluginpath.name = QT_PLUGIN_PATH
+        pluginpath.CONFIG = prepend
 
         QT_TOOL_ENV += deppath pluginpath
     }