test: always exclude devices named loop*

Not only block subsystem, but also misc has device named "loop*", and
the test always said that the following device is newly found:
---
/* test_sd_device_enumerator_filter_subsystem */
New device found: subsystem:misc syspath:/sys/devices/virtual/misc/loop-control
1 new devices are found in re-scan
---

src/libsystemd/sd-device/test-sd-device.c

@@ -215,8 +215,7 @@ static void test_sd_device_enumerator_filter_subsystem_one(
 
         assert_se(sd_device_enumerator_new(&e) >= 0);
         assert_se(sd_device_enumerator_add_match_subsystem(e, subsystem, true) >= 0);
-        if (streq(subsystem, "block"))
-                assert_se(sd_device_enumerator_add_nomatch_sysname(e, "loop*") >= 0);
+        assert_se(sd_device_enumerator_add_nomatch_sysname(e, "loop*") >= 0);
 
         FOREACH_DEVICE(e, d) {
                 const char *syspath;