[libc++][CI] Reenable disabled MacOS tests

[mordante]

Since the XCode version in the libc++ CI was updated outside of our control several tests started to fail
#135202 disabled these tests to get our CI green again. These disabled tests need to be properly evaluated and fixed.

This is a link to a working job before the issue
https://github.com/llvm/llvm-project/actions/runs/14328286272

This is a link to a failing issue that was due to the update
https://github.com/llvm/llvm-project/actions/runs/14340079784/job/40294675232?pr=134538

[mordante]

Another fix landed as e15025d

[mordante]

#135412 landed to pin the XCode version.

[ldionne]

I actually think the change that caused a failure is not an update of the Xcode version, but rather going from macOS 15.3 to macOS 15.4. If you look at the before/after jobs in the original issue description, you'll see that both are using Xcode 16.3. The difference is in the version of the operating system.