Disabling MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE reduces interoperability

[gilles-peskine-arm]

As of Mbed TLS 3.6.1, the documentation of suggests that disabling it breaks TLS 1.3 when an incompatible middlebox is disabled. But it actually breaks interoperability with any TLS 1.3 client or server that has middlebox compatibility active, such as OpenSSL or GnuTLS with default settings.

I will fix the documentation in #9546 (+ forward port). Thanks to Ronald's advice, the fix is very easy, all it takes is to adapt the tests. Done in #9563.

The definition of done for this issue is that disabling MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE only breaks interoperability when a middlebox is involved, and that it has no effect on interoperability with a peer that has middlebox compatibility enabled. This is how GnuTLS and OpenSSL behave. In particular, even when MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE is disabled, ssl_client1 and ssl_server should interoperate with GnuTLS and OpenSSL with their default settings (without requiring -no_middlebox or %DISABLE_TLS13_COMPAT_MODE).