test_git: don't hard-code "_test" to run the tests in

This paves the way to run tests in other directories, e.g. under /tmp/ .
run tests in a /tmp/ directory

These test directories do not live inside a git checkout (the versioneer
source tree itself), so they can exercise different codepaths. Namely,
what happens when 'git describe' or 'git rev-parse' cannot find a .git
directory.