Tests fail on windows

Several tests fail when executing them on a Windows system. It seems that local file paths do not get translated or processed correctly.

This is the summary for me after running `yarn test` on the freshly cloned and installed project:
```
Test Suites: 2 failed, 1 passed, 3 total
Tests: 6 failed, 6 passed, 12 total
Snapshots: 10 failed, 8 passed, 18 total
Time: 10.239s
Ran all test suites.
```

All failures seem to be related to file paths, similar to this one:
```
- [0] /node_modules/css-loader!./src/renderer/style.css {0} [built]
- [1] /node_modules/css-loader/lib/css-base.js 2.21 KiB {0} [built]"
+ [0] C:/Users/Jovica.Aleksic/Desktop/electron-webpack/node_modules/css-loader!./src/renderer/style.css {0} [built]
+ [1] C:/Users/Jovica.Aleksic/Desktop/electron-webpack/node_modules/css-loader/lib/css-base.js 2.21 KiB {0} [built]"
```

Some are failing due to mismatch in file size, but that might be just a consequence of generated files containing the longer local file paths in several occasions, e.g.:

```
- styles.css 135 bytes 0 [emitted] renderer
+ styles.css 141 bytes 0 [emitted] renderer
```

Would be great if somebody could fix the test setup so that it works on any platform.
I chatted about this with @develar once, and he said I shouldn't worry much as a user, this is rather for the core developers only.
However, writing a reply to https://github.com/electron-userland/electron-webpack/issues/197 I realized how important it is to have working tests, especially for somebody who is new and wants to contribute.
In that case, having not-really-working tests is worse than having no tests at all, because as a somewhat experienced developer, you see there are tests and naturally the first thing you do is run the tests.

In my case when I first came to this project, I spent quite some time trying to figure out what's wrong and I even approached the main developer via Slack to ask.. Because I wasn't sure if I did something wrong during project setup.
So instead of actually contributing and developing, I was "wasting time", and this may happen to other users as well.

On the other hand, having the tests simply pass - well, they are green, give you a good feeling and motivation to start hacking, and I think.. it can just be expected for the tests to work out of the box.