Fix wrong tests

Before this PR, tests were wrongly passing. `test/expected/normal.js` was wrong because it didn't include `should` module while `test/fixtures/normal.js` required `should` module.

Then, why were they passing? It was due to `should` itself. `should` extends `Object.prototype` with a property named 'should'. In the meanwhile, browserify ignores module if `self._ignore[id]` is truthy. Because of the property defined by `should`, `self._ignore['should']` is always truthy no matter what `self._ignores` contains. For this reason, `should` was never bundled by browserify.

https://github.com/substack/node-browserify/blob/ab59a9fce8faeb3b53ec581848d0bcf9dd1b58b9/index.js#L545

So, let's use `chai`'s `expect' style instead.

In addition to the fix, cleaned up tests a bit.