test_filesystem_with_cli_in_subprocess mutates os.environ a bunch

[exarkun]

It's not clear what effect these mutations may have on the test process itself but they clearly can have an effect on child processes.

There are two places this happens:

env = kwargs.get("env", os.environ)
# Python warnings from the child process don't matter.
env["PYTHONWARNINGS"] = "ignore"

and

env = os.environ
env['http_proxy'] = env['HTTP_PROXY'] = "http://127.0.0.0:12345"  # invalid address