allmydata.test.web.test_webish.TahoeLAFSSiteTests.test_large_request fails sometimes

[exarkun]

This unit test relies on removing permissions from a directory to result in an error from the platform to tell it that the code under test is operating as expected. This comes with a multi-paragraph comment to explain the weirdness.

Instead of all this, let's just change the interface of WebishServer? so it is easy to test.