add nonpoison and poison rwlock tests

Adds tests for the `nonpoison::RwLock` variant by using a macro to
duplicate the existing `poison` tests.

Note that all of the tests here are adapted from the existing `poison`
tests.

Author: connortsui20

library/std/tests/sync/lib.rs

@@ -8,6 +8,7 @@
 #![feature(std_internals)]
 #![feature(sync_nonpoison)]
 #![feature(nonpoison_mutex)]
+#![feature(nonpoison_rwlock)]
 #![allow(internal_features)]
 #![feature(macro_metavar_expr_concat)] // For concatenating identifiers in macros.