provider: load "default" provider in test_openssl_legacy_provider

Update the test case to explicitly load both the "default" and the
"legacy" providers.

Currently, the "default" provider as a side effect by the
OpenSSL::PKey::DH.new call in lib/openssl/ssl.rb. It will be cleaned up
in a following patch.

Author: rhenium

test/openssl/test_provider.rb

@@ -46,6 +46,7 @@ def test_openssl_legacy_provider
 
     with_openssl(<<-'end;')
       begin
+        OpenSSL::Provider.load("default")
         OpenSSL::Provider.load("legacy")
       rescue OpenSSL::Provider::ProviderError
         omit "Only for OpenSSL with legacy provider"