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.

Also, since permissions don't work like this on Windows, there's a branch in the test for the runtime platform.

Then, even not on Windows, permissions don't work like this if the process has extra privileges, so the test fails sometimes.

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