Zig standalone POSIX tests

This directory is just for std.posix-related test cases that depend on process-wide state like the current-working directory, signal handlers, fork, the main thread, environment variables, etc. Most tests (e.g, around file descriptors, etc) are with the unit tests in lib/std/posix/test.zig. New tests should be with the unit tests, unless there is a specific reason they cannot.