test: add workflow for golang tests

.github/workflows/test-golang.yml

@@ -0,0 +1,31 @@
+name: Test Golang
+
+# run CI on pushes to master, and on all PRs (even the ones that target other
+# branches)
+
+on:
+ push:
+   branches: [master]
+ pull_request:
+
+# set ESM_DISABLE_CACHE=true (will be JSON parsed)
+jobs:
+  gotest:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: 'true'
+    - name: Set up Go 1.15
+      uses: actions/setup-go@v1
+      with:
+        go-version: 1.15
+    - name: cache Go modules
+      uses: actions/cache@v1
+      with:
+        path: ~/go/pkg/mod
+        key: ${{ runner.os }}-go-${{ hashFiles('packages/cosmic-swingset/go.sum') }}
+        restore-keys: |
+          ${{ runner.os }}-go-
+    - name: go test
+      run: cd golang && go test ./...