Clearify running testsuite as part of patch submission

[Roland McGrath]

> I can put this in my TODO list. Would a comment in each of the test case
> files describing why this cannot be run as a regular test be
> sufficient? We can even make that part of the submission process so
> that future test cases go into xtests only if there is a valid reason
> for it.

Comments in the sources themselves, and/or in the makefile where they're
added to xtests rather than tests, are what we should have.