github/workflows: simulate a user integrating probe-engine

We want to have some confidence that a user can setup a Go repository
and use probe-engine as a library. While this seems simple, our current
dependency structure is complicated by the fact that psiphon does not
have a go.mod file, so sometimes there are breakages.

Part of #746

.github/workflows/using.yml

@@ -0,0 +1,14 @@
+name: qa
+on:
+  push:
+  schedule:
+    - cron: "14 17 * * 3"
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-go@v1
+        with:
+          go-version: "1.14"
+      - uses: actions/checkout@v2
+      - run: ./testusing.bash

testusing.bash

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+#
+# This script simulates a user creating a new project that depends
+# on github.com/ooni/probe-engine@GITHUB_SHA.
+#
+
+set -ex
+mkdir -p example.org/x
+cd example.org/x
+go mod init example.org/x
+echo > main.go << EOF
+package main
+
+import "github.com/ooni/probe-engine/libminiooni
+
+func main() {
+    libminiooni.Main()
+}
+EOF
+go get -v github.com/ooni/probe-engine@GITHUB_SHA
+go build -v .
+./x -OTunnel=psiphon -ni https://www.example.com urlgetter