wvtestrunner: catch segfaults in the subprogram.

wvtestrunner would notice if the subprogram returned a non-zero exit code,
but not if it died on a signal.  Thus, a unit test program dying on a
segfault was considered a success.  Oops!

(cherry picked from commit 9aff60fa6d55a46440a8e4d6bf343753b2bdc1ec)