Can't run tests successfully on Windows

[kfranqueiro]

When running npm test, an error occurs in the stop script:

> ./test/screenshot/stop.sh

'.' is not recognized as an internal or external command, operable program or batch file.

This might be because ps aux has different output format in MinGW on Windows. (I assume it wouldn't work at all from a standard command prompt.)

[moog16]

I think we get around this with the cross-env package in MDC-Web. I can add that and test on windows.

[lynnmercier]

Steps to reproduce:

• follow https://github.com/material-components/material-components-web-react/blob/master/docs/screenshot-tests.md#running-tests-locally on a Windows computer

Expected behavior

• local screenshots should pass

Actual behavior

• local screenshots do not pass (we need to try this again now that we've moved to Docker, but we're pretty sure our documentation does not support Windows users)

[madaz]

PR #662 includes the cross-env and rimraf changes to be able to npm run test:unit

[moog16]

@madaz we still need to get screenshot tests to work as well. This specifically references the npm stop/npm start scripts - which are run in a dockerized container. Not sure how our docker container runs on a windows machine.