Fix heartbeat_test

The heartbeat_test reaches into the internals of libssl and calls some
internal functions. It then checks the return value to check its what it
expected. However commit fa4c3745 changed the return value of these
internal functions, and now the test is failing.

The solution is to update the test to look for the new return value.

Reviewed-by: Richard Levitte <levitte@openssl.org>