avoid setEnv in test framework when tasty is running

avoid setEnv in test framework when tasty is running

setEnv is not thread safe and could cause a getEnv by another thread to
segfault, or perhaps other had behavior. This is particularly a problem
when using tasty, because tasty runs the test in a thread, and a getEnv
in another thread.

The use of top-level TMVars is ugly, but ok because only 1 test actually
runs at a time per process. Because it has to chdir into the test repo.

The setEnv that remains happens before tasty is running.

Sponsored-by: Dartmouth College's Datalad project