Skip traceme_raise* tests rather than failing upfront

This avoid reporting the tests as unexpected failure.

branches: 1.2.2;
Remove duplicated lines

Add new ptrace(2) test file

t_ptrace_sigchld - for SIGCHLD handler + ptrace(2).

Right now a single test is enabled (raise(SIGKILL)) and marked as failed
as it never finishes as the child is never collected before exiting the
parent uninterested about its child (SA_NOCLDWAIT).