[libc++] Disable std::shared_mutex benchmarks outside of dry-run

Like some of the atomic_wait benchmarks, it is very expensive and only
intended for A/B comparisons when actively investigating.

Author: ldionne

libcxx/test/benchmarks/shared_mutex_vs_mutex.bench.cpp

@@ -8,6 +8,10 @@
 
 // UNSUPPORTED: c++03, c++11, c++14
 
+// This benchmark is very expensive and we don't want to run it on a regular basis,
+// only to ensure the code doesn't rot.
+// REQUIRES: enable-benchmarks=dry-run
+
 // This benchmark compares the performance of std::mutex and std::shared_mutex in contended scenarios.
 // it's meant to establish a baseline overhead for std::shared_mutex and std::mutex, and to help inform decisions about
 // which mutex to use when selecting a mutex type for a given use case.