doc: posix: mark pthread_testcancel as supported

`pthread_testcancel()` is now implemented, mark it so.

signed-off-by: Gaetan Perrot <gaetanperrotpro@gmail.com>
diff --git a/doc/services/portability/posix/option_groups/index.rst b/doc/services/portability/posix/option_groups/index.rst
index 729bd0c..7dd4761 100644
--- a/doc/services/portability/posix/option_groups/index.rst
+++ b/doc/services/portability/posix/option_groups/index.rst
@@ -64,7 +64,7 @@
     pthread_setcanceltype(),yes
     pthread_setspecific(),yes
     pthread_sigmask(),yes
-    pthread_testcancel(),
+    pthread_testcancel(),yes
 
 .. _posix_option_group_posix_threads_ext: