mh-utils-tests: Add new tests of "folders +/"

mh-utils-tests: Add new tests of "folders +/"

* test/lisp/mh-e/mh-utils-tests.el
(mh-sub-folders-actual, mh-sub-folders): Add new tests of "folders +/".
Rewrite tests that were using 'assoc' to use 'member' instead, so that
on failure, ERT logs the list of which the element was not a member,
rather than the 'nil' returned by 'assoc'.
(mh-test-variant-handles-plus-slash): Factor out new helper function.
(mh-folder-completion-function-08-plus-slash)
(mh-folder-completion-function-09-plus-slash-tmp):
Use new helper function.
* test/lisp/mh-e/test-all-mh-variants.sh: LD_LIBRARY_PATH unnecessary.