add success message to end of run-tests script
trap EXIT, clean up test files in test scripts

fixes #32
Merge pull request #33 from ryan-williams/trap

trap EXIT, clean up test files in test scripts
set GIT_HELPERS_HOME in mk-git-test

allows this test to run correctly from an alternate copy of the
git-helpers repo, other than the one at $GIT_HELPERS_HOME.
add comment and extra checks to mk-git-test
fix overwrite-prompt logic in `mk-git`, add test