Fix tests/test_integration.py. (#808)
Fix tests/test_integration.py. (#808)

The `test_interpreter_selection_using_os_environ_for_bootstrap_reexec`
test needed to be updated to include `pex.testing`'s new `pytest`
requirement.
jsirois/pex,pantsbuild/pex,pantsbuild/pex,jsirois/pex