Add some tests for page fault signals and codes

It is useful to have some tests for page fault signals.

More tests would be useful but creating the conditions (such as various
kinds of running out of memory and I/O errors) is more complicated.

The tests page_fault_signalbus_objerr_1 and
page_fault_signalbus_objerr_2 depend on https://reviews.freebsd.org/D21566
before they can pass.

PR: 211924
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D21624