tests: fix some tests to pass in koji

FAIL: test-engine
=================
Cannot find cgroup mount point: No such file or directory
Assertion 'r >= 0' failed at ../src/test/test-engine.c:46, function main(). Aborting.

FAIL: test-sched-prio
=====================
Cannot find cgroup mount point: No such file or directory
Assertion 'r >= 0' failed at ../src/test/test-sched-prio.c:42, function main(). Aborting.

FAIL: test-bus-cleanup
======================
Assertion 'sd_bus_open_system(&bus) >= 0' failed at ../src/libsystemd/sd-bus/test-bus-cleanup.c:40, function test_bus_open(). Aborting.
after new: refcount 1

src/libsystemd/sd-bus/test-bus-cleanup.c

@@ -34,11 +34,18 @@ static void test_bus_new(void) {
         printf("after new: refcount %u\n", REFCNT_GET(bus->n_ref));
 }
 
-static void test_bus_open(void) {
+static int test_bus_open(void) {
         _cleanup_bus_unref_ sd_bus *bus = NULL;
+        int r;
 
-        assert_se(sd_bus_open_system(&bus) >= 0);
+        r = sd_bus_open_system(&bus);
+        if (r == -ECONNREFUSED || r == -ENOENT)
+                return r;
+
+        assert_se(r >= 0);
         printf("after open: refcount %u\n", REFCNT_GET(bus->n_ref));
+
+        return 0;
 }
 
 static void test_bus_new_method_call(void) {
@@ -70,11 +77,20 @@ static void test_bus_new_signal(void) {
 }
 
 int main(int argc, char **argv) {
+        int r;
+
         log_parse_environment();
         log_open();
 
         test_bus_new();
-        test_bus_open();
+        r = test_bus_open();
+        if (r < 0) {
+                log_info("Failed to connect to bus, skipping tests.");
+                return EXIT_TEST_SKIP;
+        }
+
         test_bus_new_method_call();
         test_bus_new_signal();
+
+        return EXIT_SUCCESS;
 }