Regression test for vn_io_fault

[Alan Somers]

r236321 added vn_io_fault(), a mechanism for avoiding lock order
reversals when a process reads from one file into a mmap()ed buffer
backed by another file.  From the description in the comments of
vn_io_fault() it seems like it would be possible to write a reliable
test that would trigger the LOR.  But I can't find any evidence in
svn, or bugzilla of such a test program.  None in Phabricator either,
which probably wasn't even running when that commit was made.  Did
anybody ever write a test program?  If so, I volunteer to ATFify it.

-Alan