renice: Clean up the tests a bit.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: ngie, asomers
Differential Revision: https://reviews.freebsd.org/D43087

renice: Factor out common code in tests.

Furthermore, prefer test -eq (which ignores whitespace) over a strict string equality test when comparing numerical values.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: thj
Differential Revision: https://reviews.freebsd.org/D39650

renice: Clean up the tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D39629

renice: Avoid segfault if -n flag is given without argument.

Fixes: 65ee0a8495538
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D39623

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

renice: add unit tests.

Sponsored by: Klara, Inc.