test: use lower security level in s_client
With the default security level (SECLEVEL=2), the following error
40E72B52DB7F0000:error:0A00018A:SSL routines:tls_process_ske_dhe:dh key
too small:../deps/openssl/openssl/ssl/statem/statem_clnt.c:2100
is raised on on Ubuntu 22.04 on WSL2.