tests/core/snap-set-core-config: fix test for UC24

There is no /etc/timezone file anymore on UC24.

tests/core/snap-set-core-config/task.yaml

@@ -138,9 +138,20 @@ execute: |
     sysctl -n kernel.printk|MATCH "^4\s+"
 
     echo "setting the timezone works"
-    tests.cleanup defer timedatectl set-timezone "$(cat /etc/timezone)"
+
+    get_tz() {
+        if os.query is-core-le 18; then
+            current_tz=$(cat /etc/timezone)
+        else
+            current_tz=$(timedatectl show | sed -n 's/^Timezone=//p')
+        fi
+    }
+
+    get_tz
+    tests.cleanup defer timedatectl set-timezone "$current_tz"
     snap set system system.timezone=Europe/Malta
-    MATCH "Europe/Malta" < /etc/timezone
+    get_tz
+    test "$current_tz" = "Europe/Malta"
     test "$(readlink -f /etc/localtime)" = "/usr/share/zoneinfo/Europe/Malta"
 
     echo "and timezone setting shows up in the document"
@@ -150,7 +161,8 @@ execute: |
 
     echo "and setting it again in snapd also works"
     snap set system system.timezone=America/Lima
-    MATCH "America/Lima" < /etc/timezone
+    get_tz
+    test "$current_tz" = "America/Lima"
     test "$(readlink -f /etc/localtime)" = "/usr/share/zoneinfo/America/Lima"
     
     echo "and setting the timezone outside of snapd is reflected by snap get"