test: add test for multiple inlined modules

[fi3ework]

What kind of change does this PR introduce?

Currently, there's no test case covering the following condition, if we comment this condition, no test will fail. https://github.com/webpack/webpack/blob/c586c7b1e027e1d252d68b4372f08a9bce40d96c/lib/javascript/JavascriptModulesPlugin.js#L849-L850

Did you add tests for your changes?

Adding a multiple inline modules case to cover the condition. Multiple entry file have declared same identifiers, so does non-inline modules to make sure it won't break in a more complex scenario.

Does this PR introduce a breaking change?

No.

What needs to be documented once your changes are merged?

No.

[webpack-bot]

For maintainers only:

• [ ] This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• [ ] This needs to be backported to webpack 4 (issue will be created when merged)