Improve styled-ppx.css tests

[davesnx]

While we vendored bs-css, we didn't include their test suite, which would be a nice start.

Migrate all those tests into packages/css

• https://github.com/giraud/bs-css/blob/master/bs-css/__tests__/Css_Js_test.res
• https://github.com/giraud/bs-css/blob/master/bs-css/__tests__/Selectors_test.res
• https://github.com/giraud/bs-css/blob/master/bs-css/__tests__/Svg_test.res

Ideally with the same structure of packages/emotion/test. Those test run on native, and might be beneficial of having a melange/rescript test suite as well, unsure about how it will look