[3.12] Fix ProgramPriorityTests on FreeBSD with high nice value (GH-100145) (GH-115614)

It expects priority to be capped with 19, which is the cap for Linux,
but for FreeBSD the cap is 20 and the test fails under the similar
conditions. Tweak the condition to cover FreeBSD as well.
(cherry picked from commit 437924465de5cb81988d1e580797b07090c26a28)

Co-authored-by: Dmitry Marakasov <amdmi3@amdmi3.ru>