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.