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.

Closes #3704. Closes #3687.
Merge pull request #3720 from charris/refactor-test_xerbla

MAINT: Refactor test_xerbla to hide printed error messages.