Add XCTest extensions

[AvdLee]

This PR adds two extensions that allow easy usage in XCTests:

var mock = Mock(url: url, dataType: .json, statusCode: 200, data: [.get: Data()])
let requestExpectation = expectationForCompletingMock(&mock)
let completionExpectation = expectationForCompletingMock(&mock)
mock.register()

URLSession.shared.dataTask(with: URLRequest(url: url)).resume()

wait(for: [requestExpectation, completionExpectation], timeout: 2.0)

[ghost]

	Warnings
⚠️	Consider to place some MARK: lines for Sources/Mock.swift, which is over 200 lines big.

	Messages
📖	View more details on Bitrise
📖	Mocker: Executed 20 tests, with 0 failures (0 unexpected) in 1.799 (1.887) seconds

Mocker.framework: Coverage: 89.51

File	Coverage
Mock.swift	84.62%	✅
XCTest+Mocker.swift	100.0%	✅
MockingURLProtocol.swift	86.54%	✅

Generated by 🚫 Danger Swift against 8c0e983

[Boris-Em]

Convenient

[kairadiagne]

Handy!

[ghost]

Congratulations! 🎉 This was released as part of Release 2.3.0 🚀

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