Fixture tests sometime fail because cache hasn't finished writing to disk

[aroben]

Sometimes when I run the Akavache tests I see failures like this:

Test 'Akavache.Tests.PersistentBlobCacheInterfaceFixture.CacheShouldBeRoundtrippable' failed: System.IO.IOException : The process cannot access the file 'C:\Users\Adam Roben\Documents\GitHub\Windows\Akavache\Akavache.Tests\bin\Debug\f4d14bf2-deb3-45d4-9457-f376ab69d3ea\1356c67d7ad1638d816bfb822dd2c25d' because it is being used by another process.
    at System.Reactive.ExceptionHelpers.Throw[T](Exception exception)
    at System.Reactive.Linq.Observable.FirstOrDefaultInternal[TSource](IObservable`1 source, Boolean throwOnEmpty)
    at System.Reactive.Linq.Observable.First[TSource](IObservable`1 source)
    BlobCacheFixture.cs(65,0): at Akavache.Tests.BlobCacheInterfaceFixture.CacheShouldBeRoundtrippable()

Test 'Akavache.Tests.EncryptedBlobCacheInterfaceFixture.CacheShouldBeRoundtrippable' failed: System.IO.IOException : The process cannot access the file 'C:\Users\Adam Roben\Documents\GitHub\Windows\Akavache\Akavache.Tests\bin\Debug\f901b928-26f0-474a-8408-bb9311450866\1356c67d7ad1638d816bfb822dd2c25d' because it is being used by another process.
    at System.Reactive.ExceptionHelpers.Throw[T](Exception exception)
    at System.Reactive.Linq.Observable.FirstOrDefaultInternal[TSource](IObservable`1 source, Boolean throwOnEmpty)
    at System.Reactive.Linq.Observable.First[TSource](IObservable`1 source)
    BlobCacheFixture.cs(65,0): at Akavache.Tests.BlobCacheInterfaceFixture.CacheShouldBeRoundtrippable()

It looks like the cache gets flushed to disk asynchronously and that these tests aren't waiting for it to complete before trying to use the cache files again.