C11 threads: Fix inaccuracies in testsuite

- tst-mtx-recursive.c: mtx_init fails to use mtx_plain.  Per C11
  specs, using mtx_recursive alone is not supported.  This isn't
  catched because mtx_plain is defined as 0.

- tst-thrd-sleep.c: thrd_sleep returns 0 on success, a negative
  value on failure.  Testing against thrd_success is incorrect.

- tst-tss-basic.c: tss_set is incorrectly checkd for a non-0
  value.  The test should test aginst C11 threads error codes.
  This isn't catched because thrd_success is defined as 0.

Note that all three tests fail on FreeBSD, which defines all mutex type
values, as well as all C11 threads error codes with non-0 values.

sysdeps/pthread/tst-mtx-recursive.c

@@ -27,7 +27,7 @@ do_test (void)
 {
   static mtx_t mutex;
 
-  if (mtx_init (&mutex, mtx_recursive) != thrd_success)
+  if (mtx_init (&mutex, mtx_plain | mtx_recursive) != thrd_success)
     FAIL_EXIT1 ("mtx_init failed");
 
   if (mtx_lock (&mutex) != thrd_success)

sysdeps/pthread/tst-thrd-sleep.c

@@ -27,7 +27,7 @@ static int
 sleep_thrd (void *arg)
 {
   struct timespec const *tl = (struct timespec const *) arg;
-  if (thrd_sleep (tl, NULL) != thrd_success)
+  if (thrd_sleep (tl, NULL) != 0)
     FAIL_EXIT1 ("thrd_sleep failed");
 
   thrd_exit (thrd_success);

sysdeps/pthread/tst-tss-basic.c

@@ -33,7 +33,7 @@ tss_thrd (void *arg)
   if (tss_create (&key, NULL) != thrd_success)
     FAIL_EXIT1 ("tss_create failed");
 
-  if (tss_set (key, TSS_VALUE))
+  if (tss_set (key, TSS_VALUE) != thrd_success)
     FAIL_EXIT1 ("tss_set failed");
 
   void *value = tss_get (key);