Rename testExecutorEnvs --> executorEnvs.

This was changed, but in fact, it's used for things other than tests.
So I've changed it back.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #747 from pwendell/executor-env and squashes the following commits:

36a60a5 [Patrick Wendell] Rename testExecutorEnvs --> executorEnvs.