Add a threading test for loading/unloading providers
Add a threading test for loading/unloading providers

Check that we don't see any threading issues when loading/unloading a
provider from multiple threads.

Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15010)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl