Add GitHub action for testing (google#384)

It runs all tests and confirms go.mod and go.sum are tidy, just like
the Travis CI test.

The action runs with Go 1.16 and 1.17 on darwin, linux, and windows.

.github/workflows/tests.yml

@@ -0,0 +1,19 @@
+name: Starlark Go Tests
+on: [push, pull_request]
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        go-version: [1.16.x, 1.17.x]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Install Go
+        uses: actions/setup-go@v2
+        with:
+          go-version: ${{ matrix.go-version }}
+      - name: Checkout code
+        uses: actions/checkout@v2
+      - name: Run Tests
+        shell: bash
+        run: 'internal/test.sh'

internal/test.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+# Copyright 2021 The Bazel Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+set -eu
+
+# Confirm that go.mod and go.sum are tidy
+cp go.mod go.mod.orig
+cp go.sum go.sum.orig
+go mod tidy
+diff go.mod.orig go.mod
+diff go.sum.orig go.sum
+rm go.mod.orig go.sum.orig
+
+# Run tests
+go test ./...