bin/pkill: Fix {pgrep,pkill}-j_test.sh

The POSIX sh case statement does not allow for pattern matching using the
regex + qualifier so this case statement never matches. Instead just check
for a string starting with a digit followed by any character.

While touching these files also fix various shellcheck warnings.

kyua -v parallelism=4 test failed before, succeeds now.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D28480