tests: Skip tests which need to access /sys/fs/cgroup if that is not available

Commit efdb023 ("core: unified cgroup hierarchy support") introduced a new
error ENOEXEC in cg_unified() if /sys/fs/cgroup/ is not available. Adjust the
"skip" checks in various tests accordingly.

Add a corresponding "skip" check to test-bus-creds as well, as
sd_bus_creds_new_from_pid() now calls cg_unified() as well.

This re-fixes "make check" in build chroots without /sys/fs/cgroup.

https://github.com/systemd/systemd/issues/1132

src/test/test-engine.c

@@ -38,7 +38,7 @@ int main(int argc, char *argv[]) {
         /* prepare the test */
         assert_se(set_unit_path(TEST_DIR) >= 0);
         r = manager_new(MANAGER_USER, true, &m);
-        if (IN_SET(r, -EPERM, -EACCES, -EADDRINUSE, -EHOSTDOWN, -ENOENT)) {
+        if (IN_SET(r, -EPERM, -EACCES, -EADDRINUSE, -EHOSTDOWN, -ENOENT, -ENOEXEC)) {
                 printf("Skipping test: manager_new: %s", strerror(-r));
                 return EXIT_TEST_SKIP;
         }