Summary:
Extracted basic Env-related tests from mock_env_test and memenv_test into a
parameterized test for Envs: env_basic_test.

Depends on D58449. (The dependency is here only so I can keep this series of
diffs in a chain -- there is no dependency on that diff's code.)

Test Plan: ran tests

Reviewers: IslamAbdelRahman, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D58635