Fix some test suite memory leaks

In this merge request, I'm fixing some (easy) glib test suite memory leaks. I split my changes into really simple commits, each one fixing a specific memory leak.

The most controversial commit is the one introducing g_test_cleanup as it frees some static gtestutils variables. I'm only fixing what valgrind shows me and after each commit, valgrind reports no memory leak (using glib.supp file). We can exclude that one easily if that's really important for you.

It's a first step toward fixing all test suite memory leaks and re-enabling valgrind target in .gitlab-ci.yml file (issue 333).

Hope my approach is fine.

Note that I don't understand how fedora pipeline failed while others did not.

Regards