OS-agnostic executor_pid_test alternative

[ihoro]

Currently, the utils/process/executor_pid_test.c works for FreeBSD and potentially for other BSDs, while the test suite of the project strives to be cross-platform. It opens opportunities to work on the following:

1. An OS-agnostic implementation could be considered.
2. If it's not feasible then we could think of several variations of the test covered by conditional build.
3. If it's BSD-only then a test skipping mechanism could be considered to simplify the build process and avoid Kyuafile.in patching.

The latest discussion (as of writing) could be found here: #249