Fix lshift tests

Commit 30bea14b converted bntest.c to the new TEST framework.
Unfortunately a missing "goto err" means that the lshift tests skip
the actual bit that tests them. Replacing the "goto err" reveals that
the conversion also broke the tests. This adds back the missing "goto err"
and fixes the tests.

Fixes #4808

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4809)