Re: master 7ce721b: Migrate MH-E functional tests from SourceForge

[Stephen Gildea <stepheng+emacs@gildea.com>]

>   This leads to the following test failure -- but only when
>   native-compilation is enabled.  And the backtrace looks pretty odd: It's
>   failing when trying to make a trampoline for file-directory-p?

This test mocks out both call-process and file-directory-p.  Apparently
changing file-directory-p causes call-process to be called to run a
separate Emacs process to natively compile the new file-directory-p.
This test does not expect that, and the mocked-out functions aren't
prepared to handle this case.

I can change mh-utils-tests.el to tell the native compiler to ignore it
(perhaps by binding native-comp-never-optimize-functions) but I'm
surprised that I have to.  Why should fset of a locally-bound variable
cause a separate Emacs process to be run to compile it?  That doesn't
seem to be a useful optimization, on the face of it.

Interestingly, I get a different failure when I run the tests with native
compilation enabled.  But I think the fix will be the same for both.

 < Stephen