ImageDownloaderTests reliability improvements

[1ec5]

testDownloadingImageAgainAfterFirstDownloadCompletes occasionally fails on Bitrise with an error reminiscent of #1327. Only Xcode 9.3 builds fail this way. For example, this build for #1343 failed:

testDownloadingImageAgainAfterFirstDownloadCompletes, XCTAssertTrue failed - Semaphore timed out

MapboxNavigationTests.ImageDownloaderTests
testDownloadingImageAgainAfterFirstDownloadCompletes, XCTAssertTrue failed - Semaphore timed out
/Users/vagrant/git/MapboxNavigationTests/ImageDownloaderTests.swift:120

      let secondSemaphoreResult = semaphore.wait(timeout: XCTestCase.NavigationTests.timeout)
      XCTAssert(secondSemaphoreResult == .success, "Semaphore timed out")
      

 Executed 47 tests, with 1 failure (0 unexpected) in 12.690 (12.786) seconds

/cc @akitchen

[akitchen]

@1ec5 Did this test run include the code changes in cb95112 ?

If this is indeed occurring with the URLProtocol registration changes in place, then we need to put a bit more instrumentation in place. It's also likely the case that we're not properly handling a URL loading sad path case (which can be reproduced by running the test without the URLProtocol registered).

[1ec5]

Yes, that branch is based on 31848f3 and it has failed a few times now.

[akitchen]

oof, that's frustrating. I have a few ideas as to what could be going on here, looking at other URLProtocol implementations.

Meanwhile I'll open a PR to disable the test.

[akitchen]

There were a few issues in ImageDownloaderTests and ImageDownloader itself which should be addressed in #1405

[akitchen]

The symptom described here was fixed in #1405