use default sinon-chai import

namespace import is broken, and only worked because it's the same as default import in CJS code (require()).

error TS2345: Argument of type '{ default: ChaiPlugin; }' is not assignable to parameter of type 'ChaiPlugin'.
  Type '{ default: ChaiPlugin; }' provides no match for the signature '(chai: ChaiStatic, utils: ChaiUtils): void'.

Author: dynst

packages/rxjs/spec/helpers/setup.ts

@@ -3,7 +3,7 @@
  **/
 
 import * as chai from 'chai';
-import * as sinonChai from 'sinon-chai';
+import sinonChai from 'sinon-chai';
 
 if (typeof Symbol !== 'function') {
   let id = 0;