github: Add CI for running tests

.github/workflows/tests.yml

@@ -0,0 +1,35 @@
+---
+name: Run tests
+
+on:
+  push:
+    branches-ignore: [sync, stable]
+  pull_request:
+    branches-ignore: [sync, stable]
+
+jobs:
+  run-tests:
+    name: Run tests
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    strategy:
+      matrix:
+        neovim_version: ['v0.7.0', 'v0.8.0', 'v0.9.0', 'nightly']
+
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          submodules: true
+      - run: date +%F > todays-date
+      - name: Restore cache for today's nightly.
+        uses: actions/cache@v3
+        with:
+          path: _neovim
+          key: ${{ runner.os }}-x64-${{ hashFiles('todays-date') }}
+      - name: Setup neovim
+        uses: rhysd/action-setup-vim@v1
+        with:
+          neovim: true
+          version: ${{ matrix.neovim_version }}
+      - name: Run tests
+        run: make test