tramp-test42-auto-load failure

[Juanma Barranquero <lekktu@gmail.com>]

tramp-test42-auto-load fails on Windows because remote directories are
not being tested, for lack of a suitable default. So
'tramp-test-temporary-file-directory' is set to "NUL", and
(file-remote-p "NUL") = nil.

In fact, the test used to have

   (skip-unless (tramp--test-enabled))

until commit ad99550610542f8333dc964b3bef2b9e6ad7b0f6, where it was removed.

I've sort of "fixed" it by making the test expect a failure when
'tramp--test-enabled' returns nil, because I didn't want to second
guess that commit's author, Michael Albinus.

Yet I think reverting at least part of that change would be a better
fix, because now it is "expected to fail" on Windows, while the truth
is that the test is not failing; it should simply be skipped.