Enable URLSesssion tests

[pushkarnk]

Enabling the remaining URLSession tests. These had been disabled following some CI failures that we couldn't reproduce locally. Late last month we enabled the tests with completion handlers with some extra diagnostic messages. It looks like the CI has been stable for a while (fingers crossed!). This PR enables the remaining tests that use delegates.

[pushkarnk]

@swift-ci please test

[ianpartridge]

TestFoundation/TestNSURLSession.swift:264: error: TestURLSession.test_finishTaskAndInvalidate : Asynchronous wait failed - Exceeded timeout of 12.0 seconds, with unfulfilled expectations: URLSession wasn't invalidated

[pushkarnk]

I'm looking into the failure. Doesn't look straightforward.

[pushkarnk]

Created #1049 for the failure

[pushkarnk]

@swift-ci please test

[pushkarnk]

@swift-ci please test

[pushkarnk]

One of those intermittent failures. The completion handler was called with a non-nil error which translated to URLError(_nsError: The operation could not be completed). This seems to be a very general error type - we have no info on what actually went wrong.

[pushkarnk]

I'm running the test one more time.

@swift-ci please test

[pushkarnk]

There are two types of URLSession tests - ones that use the completion handler and others that use delegates. With the tests that use delegates disabled, we seem to be doing good. However, after enabling those, the completion handler tests seem to be intermittently failing. In a debugging session today I realised that there can be some inter-test dependencies because though we are creating a new server for each test, we aren't making sure the previous server has completed and shutdown. There is a need to comprehend things better here. I am putting this PR on hold until then. I am merging this anyway but will continue investigation.