tests: make manager_skip_test() not a macro and move to tests.h

There is nothing magic in it.

src/test/test-engine.c

@@ -30,7 +30,7 @@ int main(int argc, char *argv[]) {
         assert_se(set_unit_path(get_testdata_dir()) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());
         r = manager_new(UNIT_FILE_USER, MANAGER_TEST_RUN_BASIC, &m);
-        if (MANAGER_SKIP_TEST(r))
+        if (manager_errno_skip_test(r))
                 return log_tests_skipped_errno(r, "manager_new");
         assert_se(r >= 0);
         assert_se(manager_startup(m, NULL, NULL) >= 0);