Allow qtestlib to use a "fake_now" implementation

Allow qtestlib to use a "fake_now" implementation

We then use it in test_corrupted_data() to remove an OSSL_sleep() which
may fail in some builds.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21332)