pfctl_test: avoid 200 calls to atf_get_srcdir

I have been trying to reduce the time that testsuite runs take for CheriBSD
on QEMU (currently about 22 hours). One of the slowest tests is pfctl_test:
Just listing the available test cases currently takes 98 seconds on a
CheriBSD RISC-V system due to all the processes being spawned. This trivial
patch reduces the time to 92 seconds. The better solution would be to
rewrite the test in C/C++ which I may do as a follow-up change.

Reviewed By: kp
Differential Revision: https://reviews.freebsd.org/D26417