Fix up missing Provider interfaces and refactor providerUtils.standardizeOrThrow()

contracts/test-utils/src/index.ts

@@ -22,6 +22,7 @@ export { orderUtils } from './order_utils';
 export { typeEncodingUtils } from './type_encoding_utils';
 export { profiler } from './profiler';
 export { coverage } from './coverage';
+export { Web3ProviderEngine } from '@0x/subproviders';
 export { addressUtils } from './address_utils';
 export { OrderFactory } from './order_factory';
 export { bytes32Values, testCombinatoriallyWithReferenceFuncAsync, uint256Values } from './combinatorial_utils';