tests/vfork-f.c: support platforms without vfork
tests/vfork-f.c: support platforms without vfork

On some platforms, e.g. hppa glibc, vfork() is implemented using fork
syscall, so the test cannot rely on the parent process remaining blocked
until the child process either terminates or calls execve.

* tests/vfork-f.c (main): Explicitly block the parent until the child
either terminates or calls execve.

Saruta/strace,Saruta/strace,cuviper/strace,cuviper/strace,cuviper/strace,Saruta/strace,Saruta/strace,cuviper/strace,cuviper/strace,Saruta/strace