[libc++] [test] Remove `UNSUPPORTED: stdlib=msvc` from lock.pass.cpp.

Casey Carter says, "We run std/thread/thread.mutex/thread.lock.algorithm/lock.pass.cpp
on every CI and it works fine with MSVC STL. I'm guessing this annotation predates
Billy's rewrite of std::lock in 2017-ish."

Differential Revision: https://reviews.llvm.org/D116712