test: suppress warning for already defined SSL_get_peer_certificate

test: suppress warning for already defined SSL_get_peer_certificate

build report:

    [1/2] Building C object CMakeFiles/regress.dir/test/regress_mbedtls.c.o
    /src/le/libevent/test/regress_mbedtls.c:50: warning: "SSL_get_peer_certificate" redefined
       50 | #define SSL_get_peer_certificate mbedtls_ssl_get_peer_cert
          |
    In file included from /src/le/libevent/test/regress.h:139,
                     from /src/le/libevent/test/regress_mbedtls.c:35:
    /src/oss/openssl/include/openssl/ssl.h:1799: note: this is the location of the previous definition
     1799 | #   define SSL_get_peer_certificate SSL_get1_peer_certificate
          |
    [2/2] Linking C executable bin/regress

But this should be cleaned up.