tests: Remove metaprogramming constructs for vcs test classes (issue #309):

- Removed use of the globals() and type() constructs to
programatically instantiate Git/Mercurial-specific test
classes. This should make it a bit clearer what tests are being run
at the expense of possible future VCS additions.
- Removed the SCM_TESTS VCS test configuration variable, since it got
removed. Previously it was used for instantiating test classes.
- Updated small snippet of inline documentation that described the use
of SCM_TESTS variable. New text points to inheriting from generic
test classes instead.
- base.py had a dead snippet - kill it.

remove vestiges of Python 2.5 support

We only support Python 2.6 and 2.7; hence we do not need to import
with-statement support from __future__.

tests: avoid executing tests in base classes

Fixes lots of failures with pytest by making sure base test classes don't
inherit from unittest.TestCase or by prefacing name with an underscore so tests
don't get executed by pytest.

Second step in two-part process to rename directories.
This is the actual directory rename.