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

Use .cpp as the extension for temporary C++ files.

Using a .c extension for a C++ file raises the following warning, which
breaks our header file tests if the compiler is using -Werror as well:

c++: warning: treating 'c' input as 'c++' when in C++ mode, this
behavior is deprecated

Obtained from: atf (git 3104010c2849330440cc0ce108ff341913433339)
MFC after: 3 days

Modify atf::fs::path::get_process_helpers_path API to properly
handle pathing with detail/ tests. Based on patch pushed upstream to
ATF project.

Obtained from: Garrett Cooper <yaneurabeya@gmail.com>