Put all examples in testData

[Chris Warburton]

The testData derivation contains dummy audio, arranged in dummy artist
dirs, etc. along with dummy cache data.

We should try to move all of our test inputs into this one place; for
example, if we want to test that a script warns us about AVI files, we
shouldn't just create an AVI file in that test. Instead, we should stick
some AVIs in the testData directory, since that will show us how well
all of our other scripts deal with these things.