Update test counting in checkhandshake.pm

Previously counting the number of tests in checkhandshake.pm took an
initial guess and then modified it based on various known special
cases. That is becoming increasingly untenable, so this changes it to
properly calculate the number of tests we expect to run.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2341)