add github workflow to build & test, exporting a coverage file

.github/workflows/build-test.yml

@@ -0,0 +1,22 @@
+name: Build and test
+
+on: [push, pull_request, workflow_dispatch]
+
+jobs:
+  build-test:
+    runs-on: ubuntu-latest
+    strategy:
+      max-parallel: 1 # cannot run tests concurrently, files are created
+      matrix:
+        go-version: ['stable', 'oldstable']
+    steps:
+      - uses: actions/checkout@v3
+      - uses: actions/setup-go@v4
+        with:
+          go-version: ${{ matrix.go-version }}
+      - run: make build
+      # Need to run tests with a temp dir on same file system for os.Rename to succeed.
+      - run: 'mkdir -p tmp && TMPDIR=$PWD/tmp make test'
+      - uses: actions/upload-artifact@v3
+        with:
+          path: cover.html