This is a revert of b1fcc1840c.
These tests weren't working on Ubuntu 22.04 or Fedora 37-40. I'm not
sure exactly why, but it seems like they may be incompatible with
libstdc++. Also, despite the fact that the tests were using the system
libstdc++, the tests were only run when libcxx was enabled.
I tested this with a RelWithDebInfo build and the tests passed.
Fixes #106475