renice: Force /bin/sh usage on su calls for the regression tests.

If the root account is configured to use a different shell than sh,
it fails to retreive the pid of the background process.

Approved by: des
Obtained from: chs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36930

(cherry picked from commit 07b1ea961af77d728dfef4d0a11f98a8ab2fe226)