Target main branch, ignore secret+text files in CI

The previous settings of the "Run Tests" GitHub Actions configuration
triggered builds and test runs on _every_ push on _every_ branch. This
should limit builds only to pushes on pull requests and the `main`
branch.

Also, it turns out that GitHub Actions can ignore commits that affect
only secret files or test files. So I can make README.md changes, for
example, without kicking off an unnecessary build. Pretty cool!

.github/workflows/run-tests.yaml

@@ -1,6 +1,13 @@
 name: Run Tests
 
-on: [push, pull_request, workflow_dispatch]
+on:
+  push:
+    branches: [ "main" ]
+    paths-ignore: [ ".*", "**/.*", "**.md", "**.txt" ]
+  pull_request:
+    branches: [ "main" ]
+    paths-ignore: [ ".*", "**/.*", "**.md", "**.txt" ]
+  workflow_dispatch:
 
 jobs:
   build-test: