Plugin tests: Run in separate settings path

...if no settings path is specified by the user.

The default settins path for tests will be set to the system's temporary
directory + "/qtc-${IDE_VERSION_LONG}-test-settings".

Change-Id: Ibeaf8883c80169b4d6a3a97020a16f910292d67a
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>