Fix path to the process_helpers for the libatf-c++ tests.

Because we respect the FreeBSD src tree layout under /usr/tests, and because
the layout of the tests in the atf distfile does not match the former, the
tests for atf-c++ were not able to find the process_helper binary.

Fix this by explicitly hardcoding the right path in the FreeBSD test suite.

Obtained from: atf (git 1f0e878f7f127741a3762883ef24aef317e239d5)
MFC after: 1 week