Revisions of the #778 tests, per reviewers' comments

Revisions of the #778 tests, per reviewers' comments

- Fix comments and confusing naming.
- Add tests for the new error messages suggested by David-Sarah
  and Zooko.
- Alter existing tests for new error messages.
- Make sure that the tests continue to work with the trunk.
- Add a test for a mutual disjointedness assertion that I added to
  upload.servers_of_happiness.
- Fix the comments to correctly reflect read-onlyness
- Add a test for an edge case in should_add_server
- Add an assertion to make sure that share redistribution works as it
  should
- Alter tests to work with revised servers_of_happiness semantics
- Remove tests for should_add_server, since that function no longer exists.
- Alter tests to know about merge_peers, and to use it before calling
  servers_of_happiness.
- Add tests for merge_peers.
- Add Zooko's puzzles to the tests.
- Edit encoding tests to expect the new kind of failure message.
- Edit tests to expect error messages with the word "only" moved as far
  to the right as possible.
- Extended and cleaned up some helper functions.
- Changed some tests to call more appropriate helper functions.
- Added a test for the failing redistribution algorithm
- Added a test for the progress message
- Added a test for the upper bound on readonly peer share discovery.