bugzilla 71822: Fix regression introduced to piglit-run.py

The commit 0618aa38d4a8a7c82994fb28a41576da9a2cc414 introduces a
regression when using the -t/--include-filter or -x/--exclude-filter
options. This is caused by the __iter__ magic method in the
core.Environment class returning self.__dict__.iteritems(), which
returns compiled regex objects. This will cause the json encoder to
choke.

This patch corrects the __iter__ method in core.Environment to yield
regex items as regex.pattern, and attribute storing the original string
used to create the pattern.

cc: brianp@vmware.com
cc: vlee@freedesktop.org
Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>