fixes #14155 - fix leaks between tests with random ordering

- A test for the Rabl use_controller_name_as_json_root extension was
leaking as the old value was only reset in `setup` methods. When the
test was last in the test case, the configuration wasn't reset.
- The with_env helper failed to unset environment variables that weren't
set before, it now stores a nil value and resets them.
- OS functional tests used a class-level `os` variable to hold default
parameters, but was reassigned in some tests.