eway - fix purchase test, add failure test case

use 100 for success, 105 for failure according to
https://github.com/Shopify/active_merchant/blob/da8f1ea38ed286bf2a7e3d26c67fdfbcc27bd2be/test/remote/gateways/remote_eway_rapid_test.rb
thanks @akshar-raaj and @Shopify :)