[tests] Fix itimer test warning-errors on gcc-6.4

This fixes a "suggested parens" compile warning-into-error
that shows up on gcc-6.4.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D26789

fix setitimer test for returned it_value

An old it_value of {4,3} is valid. Allow it.

Reviewed by: bdrewery
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26445

getitimer on FreeBSD returns the last set time instead of the remaining time;
test for that instead

Submitted by: pho