PG-1607 Add a very basic test for change Vault key provider

This is a very basic test but before this there were no tests at all.

contrib/pg_tde/expected/vault_v2_test.out

@@ -54,4 +54,7 @@ DROP TABLE test_enc;
 -- Creating provider fails if we can't connect to vault
 SELECT pg_tde_add_database_key_provider_vault_v2('will-not-work', :'root_token_file', 'http://127.0.0.1:61', 'secret', NULL);
 ERROR:  HTTP(S) request to keyring provider "will-not-work" failed
+-- Changing provider fails if we can't connect to vault
+SELECT pg_tde_change_database_key_provider_vault_v2('vault-v2', :'root_token_file', 'http://127.0.0.1:61', 'secret', NULL);
+ERROR:  HTTP(S) request to keyring provider "vault-v2" failed
 DROP EXTENSION pg_tde;

contrib/pg_tde/sql/vault_v2_test.sql

@@ -34,4 +34,7 @@ DROP TABLE test_enc;
 -- Creating provider fails if we can't connect to vault
 SELECT pg_tde_add_database_key_provider_vault_v2('will-not-work', :'root_token_file', 'http://127.0.0.1:61', 'secret', NULL);
 
+-- Changing provider fails if we can't connect to vault
+SELECT pg_tde_change_database_key_provider_vault_v2('vault-v2', :'root_token_file', 'http://127.0.0.1:61', 'secret', NULL);
+
 DROP EXTENSION pg_tde;