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.