MFC r314056:

Improve pjdfstest run instructions

In the Kyua era, it's no longer necessary to set PJDFSTEST_TEST_PATH. Just
use TMPDIR instead.

Reviewed by: ngie
MFC after: 3 weeks
Relnotes: yes
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9340