tests/desktop-app-info: Use unique temporary paths for action files

desktop-app-info test may fail when repeated with multiple concurrent processes because the actions test relies on checking the existence of in the shared build directory, so by doing something like:

 meson test -C _build desktop-app-info -t 0.3 --repeat 80

We may end up in timeout errors, because we are waiting for files that have been already deleted by other processes.

To avoid this, let's rely on writing the files on $G_TEST_TMPDIR env variable, that is always set and unique, given that we're using the G_TEST_OPTION_ISOLATE_DIRS test option.