Add a mean but short stress test.

I'm hoping this is a test that does some meaningful stress testing while not slowing down the test suite too much so that we can continue to run it rather than skip it.

Right now, this fails almost immediately with:

```
The process cannot access the file 'C:\dev\git\Akavache\Akavache.Tests\bin\Release\somepath\0c7b2af5aa5636eb72efa3d400accabc' because it is being used by another process.
```

Please let me know if you see a flaw in the test.