tests: always log to file during tests

And set the cachedir (for logging and other things) to a random tmpdir.

lib/utils/mu-test-utils.cc

@@ -93,7 +93,10 @@ black_hole(void)
 void
 Mu::mu_test_init(int *argc, char ***argv)
 {
+	const auto tmpdir{test_random_tmpdir()};
+
 	g_setenv("MU_TEST", "yes", TRUE);
+	g_setenv("XDG_CACHE_HOME", tmpdir.c_str(), TRUE);
 
 	g_test_init(argc, argv, NULL);