Migrate MH-E functional tests from SourceForge

Migrate MH-E functional tests from SourceForge

* test/lisp/mh-e/mh-limit-tests.el:
* test/lisp/mh-e/mh-utils-tests.el:
* test/lisp/mh-e/mh-xface-tests.el:
Import the functional tests from src/mh-unit.el.

mh-unit.el is from the old SourceForge mh-e repository, last changed
in 2017.  To this are applied unmerged patches Mike Kupfer wrote in
2018 ("Refactor mh-unit into functional and release tests" and
"Fix the functional tests").

All tests have been converted to run under Emacs's ERT framework.

Some tests for mh-utils use MH programs to examine mail folders.
These tests require an MH variant to be installed on the system; for
these, added both a mock harness that pretends the needed files and MH
programs do exist, and a wrapper that creates the necessary mail files.

New function 'mh-test-utils-setup-with-variant' bears some resemblance
to 'mh-test-folders-set-up' from the original tests.  New function
'mh-test-utils-setup-with-mocks' is new functionality for these tests.