Test errors from a provider can still be accessed after unload

Test errors from a provider can still be accessed after unload

Providers can create errors that may refer to const strings within the
provider module itself. If the provider gets unloaded we need to be sure
that we can still access the errors in the error stack.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14213)