nptl: Fix tst-pthread-gdb-attach for ptrace_scope equal 1

This is similar to the fix for elf/tst-pldd (2f9046fb):
it checks ptrace_scope value (values higher than 2 are too restrictive
to allow the test to run) and it rearranges the spawned processes
to make the target process the gdb child.

Checked on x86_64-linux-gnu with ptrace_scope set to 1.