Test case for Inferior.thread_from_thread_handle

Test case for Inferior.thread_from_thread_handle

As the title says, this is a test case for
Inferior.thread_from_thread_handle, a python method which will,
given a thread library dependent thread handle, find the GDB thread
which corresponds to that thread handle (in the inferior under
consideration).

The C file for this test case causes the thread handles for the
main thread and two child threads to be placed into an array.  The
test case runs to one of the functions (do_something()) at which point,
it retrieves the thread handles from the array and attempts to find the
corresponding thread in GDB's internal thread list.

I use barriers to make sure that both threads have actually started;
execution will stop when one of the threads breaks at do_something.

Thanks to Simon Marchi for suggestions for forcing the thread
numbering to be stable.

gdb/testsuite/ChangeLog:

	* gdb.python/py-thrhandle.c, gdb.python/py-thrhandle.exp: New
	files.