testing: pass ATF vars to pytest via env instead of arguments.

This change is a continuation of 9c42645a1e4d workaround.
Apparently pytest argument parser is not happy when parsing values
with spaces or just more than one --atf-var argument.
Switch wrapper to send these kv pairs as env variables. Specifically,
use _ATF_VAR_key=value format to distinguish from the other vars.

Add the atf_vars fixture returning all passed kv pairs as a dict.

Reviewed by: lwhsu
Differential Revision: https://reviews.freebsd.org/D35625
MFC after: 2 weeks