tests/lib/reset: try to identify unexpected snap units still present …

…after purge

The reset code runs purge, but give how purge works, it may happen that there
are unexpected snap units. Specifically, lxd tends to start
snap.lxd.workaround.service unit directly through systemd-run, which is not
known to snapd and thus will not be stopped during purge.

This should fix the following issue in tests/main/postrm-purge:

```
 + systemctl --plain --no-legend --full
+ grep -E 'snap\..*\.(service|timer|socket)'
snap.lxd.workaround.service                                                                  loaded active exited    /bin/true
+ echo 'found unexpected leftovers'
found unexpected leftovers
+ exit 1
```

Signed-off-by: Maciej Borzecki <[email protected]>

tests/lib/reset.sh

@@ -35,6 +35,23 @@ reset_classic() {
             ;;
     esac
 
+    local unexpected_units=0
+    for unit in $(systemctl --plain --no-legend --full | awk '/^ *snap\..*\.service +loaded/ {print $1}'); do
+        case "$unit" in
+            snap.lxd.workaround.service)
+                systemctl stop "$unit" || true
+                ;;
+            *)
+                echo "unexpected unit $unit"
+                unexpected_units=1
+                ;;
+        esac
+    done
+    if [ "$unexpected_units" != "0" ]; then
+        echo "error: found unexpected systemd units after purge"
+        exit 1
+    fi
+
     # purge may have removed udev rules, retrigger device events
     udevadm trigger
     udevadm settle