TEST: Add provider_fallback_test, to test aspects of fallback providers

There are cases where the fallback providers aren't treated right.
For example, the following calls, in that order, will end up with
a failed EVP_KEYMGMT_fetch(), even thought the default provider
does supply an implementation of the "RSA" keytype.

    EVP_KEYMGMT *rsameth = NULL;

    OSSL_PROVIDER_available(NULL, "default");
    rsameth = EVP_KEYMGMT_fetch(NULL, "RSA", NULL);

For good measure, this also tests that explicit loading of the default
provider won't fail.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11926)