Fix linking for multi-source file tests, the second part of the tuple
needs to be the C source file (not the base name)

new tests for posix_spawn_chdir from Piyush Sachdeva

Add a few more posix_spawn testcases, which should detect everything that
was wrong in the initial version, causing the failure reported in
PR kern/45991.

Move posix_spawn tests to lib/libc/gen/posix_spawn - they test both libc
and kernel, but that is an implementation detail unrelated to the tests
themselfs.
Ok: releng