make sure minitest does not install it's exit hook

we run minitest tests directly so we need to make sure that minitest
does not hook into exit by default. This should only happen when
minitest/autorun is loaded but this seems to be the case for the rails
default setup outside of the user controlled test_helper or
minitest_helper
Merge pull request #348 from sideshowcoder/minitest_multiple_runs_fix

make sure minitest does not install it's exit hook