Add test adapter and interpolate secrets in integration tests

lib/kamal/secrets/adapters/test.rb

@@ -0,0 +1,10 @@
+class Kamal::Secrets::Adapters::Test < Kamal::Secrets::Adapters::Base
+  private
+    def login(account)
+      true
+    end
+
+    def fetch_from_vault(secrets, account:, session:)
+      secrets.to_h { |secret| [ secret, secret.reverse ] }
+    end
+end