Add tests for logError thread-safety / Make it return a value

[mokagio]

See @salimbraksa's comment from Jetpack and WordPress iOS: https://github.com/wordpress-mobile/WordPress-iOS/pull/20846/files#r1231271865

I could only find one usage of logError in the tests, and is commented:

Automattic-Tracks-iOS/Tests/Tests/CrashLoggingTests.swift

Lines 97 to 114 in 01e7397

	///
	/// These are currently disabled, but are being left here, because I'm hoping to get
	/// back to this and sort out how to send stack traces for these events.
	///
	// func testThatLoggedErrorsContainAStackTrace() {
	// let expectation = XCTestExpectation(description: "Event should contain a Stack Trace")
	//
	// mockDataProvider.didLogErrorCallback = { event in
	// expectation.isInverted = event.stacktrace == nil // fail if `stacktrace` is nil
	// expectation.fulfill()
	// }
	//
	// CrashLogging.start(withDataProvider: mockDataProvider)
	// CrashLogging.logError("This is a test")
	//
	// wait(for: [expectation], timeout: 1)
	// }