Fix VyOS check mode (#35977)

* Revert "commented out check tests (#35788)"

This reverts commit 3df2561405.

* `discard_changes()` now uses `exit discard`

instead of just `discard`. Also a slight tweak to avoid a second `exit` after
`exit discard`

test/integration/targets/vyos_smoke/tests/cli/misc_tests.yaml

@@ -1,13 +1,13 @@
 # hit check conditional in module_utils.network.vyos -> load_config()
-# - name: configure simple config command
-#   vyos_config:
-#     lines: set system host-name check-test
-#   check_mode: yes
-#
-# - name: get host name
-#   vyos_command:
-#     commands: show host name
-#   register: result
-#
-# - assert:
-#     that: '"check-test" not in result.stdout'
+- name: configure simple config command
+  vyos_config:
+    lines: set system host-name check-test
+  check_mode: yes
+
+- name: get host name
+  vyos_command:
+    commands: show host name
+  register: result
+
+- assert:
+    that: '"check-test" not in result.stdout'