Add a test to ssl-opt.sh to ensure that in DTLS a 6 byte record counter
is compared in ssl_check_ctr_renegotiate() instead of a 8 byte one as in
the TLS case. Because currently there are no testing facilities to check
that renegotiation routines are triggered after X number of input/output
messages, the test consists on setting a renegotiation period that
cannot be represented in 6 bytes, but whose least-significant byte is 2.
If the library behaves correctly, the renegotiation routines will be
executed after two exchanged.
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