test: use valid mock api key prefix (pk_test_)

Author: Albert

spec/altertable_spec.rb

@@ -3,7 +3,7 @@
 require "spec_helper"
 
 RSpec.describe Altertable do
-  let(:api_key) { "test_pk_abc123" }
+  let(:api_key) { "pk_test_123" }
   let(:base_url) { "http://127.0.0.1:15000" }
 
   before do