[CI] add workflow_dispatch hook to bypass GitHub fork-PR throttling

The new module addition (dms_libreoffice_preview) pushed after a burst
of activity on PR #3 / PR #5 today \u2014 GitHub's anti-abuse heuristic
silently dropped the pull_request trigger for PR #6. workflow_dispatch
lets us `gh workflow run` manually when this happens. Same pattern
used on the 19.0-imp-dms-ux + 19.0-ci-ui-artifacts branches.

Author: dnplkndll

.github/workflows/test.yml

@@ -8,6 +8,10 @@ on:
     branches:
       - "19.0"
       - "19.0-ocabot-*"
+  # ledoent fork only: allow manual re-trigger when GitHub silently throttles
+  # fork-PR workflow runs after a burst of pushes. Strip before opening an
+  # upstream OCA PR (oca-addons-repo-template owns this file).
+  workflow_dispatch:
 
 jobs:
   unreleased-deps: