bug#51902: 28.0.50; [PATCH] Get mh-e tests to pass

[Stephen Gildea]

Mike, bug report https://debbugs.gnu.org/51902 causes me to notice that
our fix for "+/" works only with nmh.  It fails with GNU Mailutils and
with MH mocked out.

I can fix the mocks (and am sad I didn't think to test that before
committing the patch).

But what about mailutils?  I think you told me that you had to get a
bug in nmh fixed before this would work; perhaps mailutils has the same
bug?  If so, do we need to make an exception in the test as suggested
by this bug report: expect failure with mailutils?

 < Stephen