There are various existing tests that call pthread_attr_init and then
verify properties of the resulting initial values retrieved with
pthread_attr_get* functions. However, those are missing coverage of
the initial values retrieved with pthread_attr_getschedparam and
pthread_attr_getstacksize. Add testing for initial values from those
functions as well.
(tst-attr2 covers pthread_attr_getdetachstate,
pthread_attr_getguardsize, pthread_attr_getinheritsched,
pthread_attr_getschedpolicy, pthread_attr_getscope. tst-attr3 covers
some of those together with pthread_attr_getaffinity_np.
tst-pthread-attr-sigmask covers pthread_attr_getsigmask_np.
pthread_attr_getstack has unspecified results if called before the
relevant attributes have been set, while pthread_attr_getstackaddr is
deprecated.)
Tested for x86_64.
