tests: clear error queue before executing a testcase

tests: clear error queue before executing a testcase

There can be errors in the queue from previous tests and
we look at it to verify we do not add spurious errors in
some testcases.

Fixes #19477

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19483)

(cherry picked from commit 4bae06d47ae26b37a948d31f11884e1813f6d669)