Add test for consuming s2n-tls with CMake via FetchContent

[goatgoose]

Problem:

#5076 adds support for consuming s2n-tls via the FetchContent CMake feature. We should make sure that we don't have a regression which breaks this build.

Solution:

A new test should be added which builds a CMake project that consumes s2n-tls with FetchContent. This will ensure we continue supporting this use case in the future.

We may be able to borrow from @jmayclin's consume-s2n-tls repo.