ci: stop double-running test workflows on PR branches (#181)

All seven test-* workflows triggered on both push and pull_request with no
branch filter, so every push to a PR branch fired both events: two runs each.
Restrict push to main: PR branches now trigger only pull_request, and a merge to
main triggers only push. pull_request stays unfiltered so PRs against any base
still run. Roughly halves Actions usage for these workflows.

Author: reuvenharrison

.github/workflows/test-breaking.yaml

@@ -1,7 +1,8 @@
 name: breaking
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   oasdiff_breaking:
     runs-on: ubuntu-latest

.github/workflows/test-changelog.yaml

@@ -1,7 +1,8 @@
 name: changelog
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   oasdiff_changelog:
     runs-on: ubuntu-latest

.github/workflows/test-diff.yaml

@@ -1,7 +1,8 @@
 name: diff
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   oasdiff_diff:
     runs-on: ubuntu-latest

.github/workflows/test-error-annotation.yaml

@@ -1,7 +1,8 @@
 name: error-annotation
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   entrypoint_error_annotation:
     runs-on: ubuntu-latest

.github/workflows/test-pr-comment.yaml

@@ -1,7 +1,8 @@
 name: pr-comment
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   pr_comment_tolerates_oasdiff_fail_on:
     runs-on: ubuntu-latest

.github/workflows/test-validate.yaml

@@ -1,7 +1,8 @@
 name: validate
 on:
-  pull_request:
   push:
+    branches: [main]
+  pull_request:
 jobs:
   oasdiff_validate_valid:
     runs-on: ubuntu-latest

.github/workflows/test-verify.yaml

@@ -1,6 +1,7 @@
 name: test verify
 on:
   push:
+    branches: [main]
   pull_request:
 
 jobs: