Update SystemdService.exists to find all unit files

Author: CarstenGrohmann

testinfra/modules/service.py

@@ -176,12 +176,18 @@ def _has_systemd_suffix(self):
 
     @property
     def exists(self):
-        cmd = self.run_test('systemctl list-unit-files | grep -q "^%s"', self.name)
-        return cmd.rc == 0
+        # systemctl return codes based on https://man7.org/linux/man-pages/man1/systemctl.1.html:
+        # 0: unit is active
+        # 1: unit not failed (used by is-failed)
+        # 2: unused
+        # 3: unit is not active
+        # 4: no such unit
+        cmd = self.run_expect([0, 1, 3, 4], "systemctl status %s", self.name)
+        return cmd.rc < 4
 
     @property
     def is_running(self):
-        # based on https://man7.org/linux/man-pages/man1/systemctl.1.html
+        # systemctl return codes based on https://man7.org/linux/man-pages/man1/systemctl.1.html:
         # 0: program running
         # 1: program is dead and pid file exists
         # 3: not running and pid file does not exists