Refactor txaws.s3.test.test_client

[Drew Smathers]

Many of the test cases for s3 client define a local StubQuery class which does some assertions in its __init__() method. There are potential some problems:

1. I'm not sure how many meaningful most of the assertions are there (just verifying basic attributes are set)... perhaps its just cruft that could be eliminated?
2. If we like this testing style perhaps it could be more DRY (*)

(*) By this I mean a global test class that can either be extended with post_init() method for housing assertions or take a callback in its constructor for doing post init assertions.