Add regress tests for switching from the new TLS stack to the legacy stack.

This includes a test where the server response includes multiple handhshake
messages in the single TLS plaintext record (w

Add regress tests for switching from the new TLS stack to the legacy stack.

This includes a test where the server response includes multiple handhshake
messages in the single TLS plaintext record (which would have caught the
bug just fixed in tls13_legacy.c).

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