Create unit and end-to-end tests to test background session logic

[bhamiltoncx]

Currently, the only tests around APIs like +[GTMSessionFetcher fetcherWithSessionIdentifier:] and +[GTMSessionUploadFetcher uploadFetcherForSessionIdentifier:] are manual tests.

To test the functionality, you have to manually change #define values like these:

gtm-session-fetcher/TestApps/FetcheriOSTestApp/FTAIOSAppDelegate.m

Line 30 in dc9988c

#define ENABLE_OUT_OF_PROCESS_DOWNLOAD_TESTING 0

and build, run, kill, and re-launch various test apps.

We should instead:

1. Create a @protocol to model the session-based logic,
2. Create a fake implementation for tests, and
3. Create a real implementation which forwards the UIApplication calls in

This allows writing unit tests for this logic, but we should also make end-to-end tests. For that, we can use XCUITest to install, kill, and restart a test host app.