Add empty record tests

The previous commit changed how we handle out-of-context empty records.
This commit adds some tests for the various scenarios. There are three
tests:
1: Check that if we inject an out-of-context empty record then we fail
2: Check that if we inject an in-context empty record then we succeed
3: Check that if we inject too many in-context empty records then we fail.

Reviewed-by: Andy Polyakov <appro@openssl.org>