tests: support running tests in github agents with spread-enabled tag (…

…#14210)

This is a required step to allow to move snapd project under canonical
org.
In canonical org we have shared github action runners with no spread
installed. So, by using this tag we make sure for testing stepr we run
on runners with spread installed.

.github/workflows/test.yaml

@@ -618,7 +618,7 @@ jobs:
     # release branches
     if: ${{ github.event_name != 'push' || github.ref != 'refs/heads/master' }}
     name: ${{ matrix.group }}
-    runs-on: self-hosted
+    runs-on: [self-hosted, spread-enabled]
     strategy:
       # FIXME: enable fail-fast mode once spread can cancel an executing job.
       # Disable fail-fast mode as it doesn't function with spread. It seems