MAINT: Refactor test_xerbla to hide printed error messages.

Combine the xerbla override tests with the previous _is_xerbla_safe
decorator. The xerbla checks are then made in a forked process where the
outputs are closed so that if xerbla prints a message it does not
appear.

It would be nice to figure out a Windows version of this test.
Merge pull request #3721 from charris/backport-1.8-3720

Backport 1.8 3720