tests: make sure tests pass when invoked in "sudo"

This is a follow-up for 6309e51ea3 and makes sure
we compare test results with the right user identifier.

src/test/test-unit-file.c

@@ -589,7 +589,7 @@ static void test_install_printf(void) {
         assert_se(specifier_machine_id('m', NULL, NULL, &mid) >= 0 && mid);
         assert_se(specifier_boot_id('b', NULL, NULL, &bid) >= 0 && bid);
         assert_se((host = gethostname_malloc()));
-        assert_se((user = getusername_malloc()));
+        assert_se((user = uid_to_name(getuid())));
         assert_se(asprintf(&uid, UID_FMT, getuid()) >= 0);
 
 #define expect(src, pattern, result)                                    \