fix(test): Avoid EPIPE error in "web-ext sign" test

[Rob--W]

In our fake AMO server, we are immediately sending a response without consuming the HTTP request body. This code pattern is known to cause EPIPE failures (see nodejs/node#12339). The fix is to consume the request body before responding.

Minimal local reproduction (tested with Node.js v10.6.0):

path/to/web-ext/tests/functional/fake-amo-server.js

$ node -e 'require("http").request({port:8989,method:"POST"},res=>{console.log(res.statusCode); res.pipe(process.stdout)}).end("x".repeat(1E7))'
200
{"url":"http://localhost:8989/validation-results/"}events.js:167
      throw er; // Unhandled 'error' event
      ^

Error: write EPIPE
    at WriteWrap.afterWrite [as oncomplete] (net.js:835:14)
Emitted 'error' event at:
    at Socket.socketErrorListener (_http_client.js:382:9)
    at Socket.emit (events.js:182:13)
    at onwriteError (_stream_writable.js:431:12)
    at onwrite (_stream_writable.js:456:5)
    at _destroy (internal/streams/destroy.js:40:7)
    at Socket._destroy (net.js:605:3)
    at Socket.destroy (internal/streams/destroy.js:32:8)
    at WriteWrap.afterWrite [as oncomplete] (net.js:837:10)

After this patch, the above error does not occur any more. I have also verified that GET requests also complete successfully.

[coveralls]

Coverage remained the same at 100.0% when pulling 3b560fd on Rob--W:bye-bye-EPIPE into 4134812 on mozilla:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 3b560fd on Rob--W:bye-bye-EPIPE into 4134812 on mozilla:master.

[rpl]

@Rob--W the change looks great, and that was also my guess.

Let's merge this! \o/

👋 "bye bye EPIPE error" ;-)