ci: add workflow file for GitHub action

Must do this on master, or it won't run. Not supposed to pass (because
PG16 is not supported and because it'll need to install pgmp-dev)

.github/workflows/tests.yml

@@ -0,0 +1,26 @@
+name: Tests
+
+on:
+  push:
+    # This should disable running the workflow on tags, according to the
+    # on.<push|pull_request>.<branches|tags> GitHub Actions docs.
+    branches:
+      - "*"
+  pull_request:
+
+concurrency:
+  # Cancel older requests of the same workflow in the same branch.
+  group: ${{ github.workflow }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    strategy:
+      matrix:
+        pg: [12, 11, 10, 9.6, 9.5, 9.4, 9.3, 9.2]
+    runs-on: ubuntu-latest
+    container: pgxn/pgxn-tools
+    steps:
+      - run: pg-start ${{ matrix.pg }}
+      - uses: actions/checkout@v3
+      - run: pg-build-test