Add a fallback test workflow. (#2971)

To make this cleaner, this switches the main `test` workflow to use
exclusion path patterns as that was the clearly documented intent
anyways.

This works around a problem with required GitHub status checks:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

Without something like this, we won't be able to use the merge queue (or
require passing tests).

.github/workflows/tests.yaml

@@ -7,32 +7,23 @@ name: test
 on:
   push:
     branches: [trunk]
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
   pull_request_target:
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
   merge_group:
-    paths:
-      # Conservatively run the tests. However, skip them if the only paths in
-      # the pull request match files that we know don't impact the build.
-      - '**'
-      - '!**/*.md'
-      - '!LICENSE'
-      - '!CODEOWNERS'
-      - '!.git*'
+    paths-ignore:
+      - '**/*.md'
+      - 'LICENSE'
+      - 'CODEOWNERS'
+      - '.git*'
 
 # Cancel previous workflows on the PR when there are multiple fast commits.
 # https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency