nixos/lib/test-driver: make wait_for_unit ask for ActiveState only

This is a hotfix for https://github.com/NixOS/nixpkgs/issues/266690,
where `systemctl show $unit` sometimes randomly leaves ActiveState out

nixos/lib/test-driver/test_driver/machine.py

@@ -447,8 +447,7 @@ class Machine:
         """
 
         def check_active(_: Any) -> bool:
-            info = self.get_unit_info(unit, user)
-            state = info["ActiveState"]
+            state = self.get_unit_property(unit, "ActiveState", user)
             if state == "failed":
                 raise Exception(f'unit "{unit}" reached state "{state}"')
 
@@ -491,6 +490,35 @@ class Machine:
             if line_pattern.match(line)
         )
 
+    def get_unit_property(
+        self,
+        unit: str,
+        property: str,
+        user: Optional[str] = None,
+    ) -> str:
+        status, lines = self.systemctl(
+            f'--no-pager show "{unit}" --property="{property}"',
+            user,
+        )
+        if status != 0:
+            raise Exception(
+                f'retrieving systemctl property "{property}" for unit "{unit}"'
+                + ("" if user is None else f' under user "{user}"')
+                + f" failed with exit code {status}"
+            )
+
+        invalid_output_message = (
+            f'systemctl show --property "{property}" "{unit}"'
+            f"produced invalid output: {lines}"
+        )
+
+        line_pattern = re.compile(r"^([^=]+)=(.*)$")
+        match = line_pattern.match(lines)
+        assert match is not None, invalid_output_message
+
+        assert match[1] == property, invalid_output_message
+        return match[2]
+
     def systemctl(self, q: str, user: Optional[str] = None) -> Tuple[int, str]:
         """
         Runs `systemctl` commands with optional support for