Fix headless test runner on macOS High Sierra

[LegNeato]

No description provided.

[alexcrichton]

Thanks for this! Can you detail a bit about what the bug was? I'm not entirely sure what #[serde(flatten)] is doing here but it seems like it's fixing the issue?

[LegNeato]

Sure! Apparently post High Sierra the format of safaridriver's response changed. On my machine (High Sierra), I get:

{
  "status": 0,
  "sessionId": "7E00E58A-859E-4127-9E0A-F67FBE4CB647",
  "value": {
    "version": "13605.3.8",
    "rotatable": false,
    "applicationCacheEnabled": true,
    "databaseEnabled": true,
    "handlesAlerts": true,
    "cleanSession": true,
    "browserName": "safari",
    "javascriptEnabled": true,
    "locationContextEnabled": false,
    "platform": "macOS",
    "webStorageEnabled": true,
    "cssSelectorsEnabled": true,
    "nativeEvents": true,
    "platformName": "macOS"
  }
}

Because Option::or is eager, for the case where a value entry exists but doesn't contain sessionId, deserializing would fail.

serde(flatten) is supposed to hoist the keys of the substruct into the parent struct, but after manually testing I see it doesn't behave the way I want for this fix. So I pushed a new change that should work for all cases.

In this playground link, you can see the three cases:

• sessionId is at the top level with a value entry with no sessionId ("--legacy" or High Sierra and below)
• sessionId is at the top level without a value entry
• no sessionId at the top level but one in value

[alexcrichton]

Ah I see, thanks for the explanation! The PR as-is definitely makes sense to me as well!