Update testtools.compat.__all__.

Remove references to functions that have been removed in
deedadf3750c16c2f26c5680a158ac96ca10ccd6 from __all__.
Merge pull request #117 from jelmer/all-all

Update testtools.compat.__all__.
Depend on unittest2

For now the main change this brings that that we inherit it's discovery
functionality and the ``TestProgram`` base class.

This brings in many fixes made to discovery where previously we were only using
the discovery package or the version in the release of Python that the test
execution was occuring on. (Robert Collins, #1271133)

Change-Id: I6270c8e8949262c01cb3a4e40735077ad6fc1ef2