Fix test pollution and unit tests.
What was happening was that a single test failure would cause all following tests to fail with directory not found when trying to modify the home directory.

This also uncovered an issue with test_diff_exclude_in_subdir and test_diff_exclude_root_pattern_in_subdir
Also I'm removing the mktemp since it is deprecated and the class already provides that functionality.

time ./tests/runtests.py
/Volumes/Augie/solarmist/rbtools/lib/python2.7/site-packages/nose/pyversion.py:36: DeprecationWarning: The 'new' module has been removed in Python 3.0; use the 'types' module instead.

  import new

/Volumes/Augie/solarmist/rbtools/lib/python2.7/site-packages/nose/plugins/prof.py:14: DeprecationWarning: The 'hotshot' module is not supported in 3.x, use the 'profile' module instead.

  import hotshot

/Volumes/Augie/solarmist/rbtools/lib/python2.7/site-packages/nose/pyversion.py:49: DeprecationWarning: Overriding __eq__ blocks inheritance of __hash__ in 3.x

  class Key(object):

#1 Testing the cache with a high max-age value ... /Volumes/Augie/solarmist/rbtools/rbtools/api/cache.py:479: DeprecationWarning: buffer() not supported in 3.x

  sqlite3.Binary(entry.response_body)))

ok

#2 Testing the cache with a zero max-age value ... /Volumes/Augie/solarmist/rbtools/rbtools/api/cache.py:493: DeprecationWarning: buffer() not supported in 3.x

  sqlite3.Binary(entry.response_body), entry.url,

ok

...

Ran 210 tests in 204.074s
OK

./tests/runtests.py  85.38s user 56.70s system 69% cpu 3:24.53 total