CI - C#/Unity test suite concurrency improvements (#3001)

# Description of Changes

New Unity test suite runs will cancel existing in-progress runs on the
same PR or github ref.

I tried to add a global limit of 2 test suites running at the same time,
but I did not find a successful way of making this work.

# API and ABI breaking changes

None. CI-only change.

# Expected complexity level and risk

1

# Testing
- [x] Test suite still runs and succeeds
- [x] If I push several commits in a row, previous runs get cancelled

---------

Co-authored-by: Zeke Foppa <[email protected]>

Author: bfops

.github/workflows/csharp-test.yml

@@ -9,6 +9,10 @@ on:
 jobs:
   unity-testsuite:
     runs-on: ubuntu-latest
+    # Cancel any previous testsuites running on the same PR and/or ref.
+    concurrency:
+      group: unity-test-${{ github.event.pull_request.number || github.ref }}
+      cancel-in-progress: true
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4