Small fixes to test suite in the hope that it will facilitate the
adoption of TestCaseWithMemoryTransport.

* bzrlib/tests/test_selftest.py:
(TestTestCaseWithMemoryTransport.test_make_branch_and_memory_tree,
TestTestCaseWithMemoryTransport.test_make_branch_and_memory_tree_with_format)):
Ensures that MemoryTransport is not leaking.

* bzrlib/tests/__init__.py:
(TestCaseWithMemoryTransport.failUnlessExistsOnDisk,
TestCaseWithMemoryTransport.failIfExistsOnDisk): Renamed from
failUnlessExists and failIfExists which should remain targeted at
testing that a file do not exists in the context of the transport
specific test.
(TestCaseInTempDir.failUnlessExists,
TestCaseInTempDir.failIfExists): Lazily maintain compatibility
with existing tests: assumes that we work in the current directory
*on disk*.