[libc++] Fix copy/pasta error in atomic tests for `atomic_compare_exchange_{weak,strong}` (#87135)

Spotted this minor mistake in the tests as I was looking into testing
more thoroughly `atomic_ref`.

The two argument overloads are tested just above. The names of the
lambda clearly indicates that the intent was to test the one argument
overload.

GitOrigin-RevId: 4078763e2e73b4ef3f9e728f66cdf9e429d3f7a4
2 files changed