Add ability to run tests on remote webdriver.

[Kirguir]

Resolve #1731, #1732

• added parsing env vars like GECKODRIVER_REMOTE for address of remote webdriver
• removed parsing env vars like GECKODRIVER_CLIENT_ARGS
• added parsing a file webdriver.json for configure browser's capabilities, like:

{
  "moz:firefoxOptions": {
    "prefs": {
      "media.navigator.streams.fake": true,
      "media.navigator.permission.disabled": true,
    },
    "args": []
  },
  "goog:chromeOptions": {
    "args": [
      "--use-fake-device-for-media-stream",
      "--use-fake-ui-for-media-stream"
    ]
  }
}

[alexcrichton]

Thanks for this and sorry for the delay in reviewing! This is getting complicated enough that would you mind writing up a page in guide/* about all the various options to configure this? Also is the webdriver dependency here necessary, or could we perhaps continue to use raw json blobs using just serde?

[Kirguir]

You can use a raw JSON blob, as shown in the example - each browser will only process its own section of options.
Of course I will add a configuration description to this guide.

[alexcrichton]

Thanks! And I'm just thinking that the webdriver dependency isn't really buying us much since it's just a few structures, so would it be possible to remove the dependency on it? Additionally do you know if there's documentation for safari's supported options?

[Kirguir]

For Safari, unfortunately, did not find anything. Do you mean remove webdriver from here?

[alexcrichton]

Ah ok, seems fine to start out with chrome/firefox in any case. And yeah by removing webdriver I mean the crate used as a dependency.

[alexcrichton]

Looks great to me, thanks!