tests: posix: headers: add verification tests for more constants

Ensure that the "headers" test checks that the following constants
are defined:

* PTHREAD_PROCESS_SHARED
* PTHREAD_PROCESS_PRIVATE
* PTHREAD_COND_INITIALIZER
* PTHREAD_MUTEX_INITIALIZER
* PTHREAD_CANCELED

They were already defined by previous commits, but the test had not
been updated.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>