feat: Allow triggering test and validate workflow manually

It will be easier debugging issues on checks failing on PRs by running
triggering them on the default branch, to rule out if the problem is
only in the PR or also on the default branch.

Documentation:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

Part of #395

Author: Erik Westrup

.github/workflows/test.yml

@@ -1,5 +1,6 @@
 name: Test
 on:
+  workflow_dispatch:
   # Ensure GitHub actions are not run twice for same commits
   push:
     branches: [master]

.github/workflows/verify-go-src.yml

@@ -1,6 +1,7 @@
 name: Verify Go client
 
 on:
+  workflow_dispatch:
   push:
     branches: [master]
   pull_request: