Test attaching to a program that constantly spawns short-lived threads

Before the previous fixes, on Linux, this would trigger several
different problems, like:

 [New LWP 27106]
 [New LWP 27047]
 warning: unable to open /proc file '/proc/-1/status'
 [New LWP 27813]
 [New LWP 27869]
 warning: Can't attach LWP 11962: No child processes
 Warning: couldn't activate thread debugging using libthread_db: Cannot find new threads: debugger service failed
 warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.

gdb/testsuite/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* gdb.threads/attach-many-short-lived-threads.c: New file.
	* gdb.threads/attach-many-short-lived-threads.exp: New file.