testsuite/threads/: On Wim's request, split the test in three separately-compiled tests that each test a very specifi...

Original commit message from CVS:
* testsuite/threads/Makefile.am:
* testsuite/threads/threadi.c: (cb_data), (cb_play), (main):
On Wim's request, split the test in three separately-compiled
tests that each test a very specific bug. Two of them still fail,
will create bugs for those. threadi.c indicates why they fail.