Add github workflow that runs buttercup on push and pull requests

.github/workflows/test.yml

@@ -0,0 +1,25 @@
+name: test
+on:
+  push: {branches: [master]}
+  pull_request: {branches: [master]}
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        emacs_version:
+          - '26.3'
+          - 'snapshot'
+        include:
+          - emacs_version: 'snapshot'
+            allow_failure: true
+    steps:
+      - uses: actions/checkout@v2
+      - uses: purcell/setup-emacs@master
+        with:
+          version: ${{ matrix.emacs_version }}
+      - uses: conao3/setup-cask@master
+
+      - name: Run tests
+        run: "cask && cask exec buttercup -L . || ${{ matrix.allow_failure == true }}"