textlib.extract_templates_and_params tests

allow textlib.extract_templates_and_params_mwpfh to be
accessed and tested even if config.mwparserfromhell is disabled.

Delay import of mwparserfromhell until it is actually used.

Merge common test results for mwpfh and regex, highlighting the
case known to be different, and mention them in the docstring.

Change-Id: Id6a17940f241d95d9e8dc9b86131dec3989ea36a
Merge "textlib.extract_templates_and_params tests"