Fix test_wsgiref.testEnviron() to no longer depend on the environment
variables (don't fail if "X" variable is set).

testEnviron() now overrides os.environ to get a deterministic
environment. Test full TestHandler.environ content: not only a few
selected variables.
(cherry picked from commit 5150d327)
Co-authored-by: Victor Stinner <vstinner@redhat.com>