test: sourcemaps with an actual plugin

[romainmenke]

This PR contains a:

• [ ] bugfix
• [ ] new feature
• [ ] code refactor
• [x] test update
• [ ] typo fix
• [ ] metadata update

Motivation / Use-Case

PostCSS has two modes:

• LazyResult
• NoWorkResult

NoWorkResult is an optimized path that is taken if no plugins, parsers, ... are provided. The assumption here is that the user has a bad config and doesn't actually want to use PostCSS.

LazyResult is PostCSS actually doing things.

The sourcemaps generated for both modes are radically different.

This changes adds a noop plugin to trigger the LazyResult mode.

Breaking Changes

N/A

Additional Info

I don't know if this might surface subtle bugs here. Main purpose is to have better test coverage.

I am trying to pinpoint the cause of this issue : https://github.com/postcss/postcss/issues/1914

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: romainmenke / name: Romain Menke (4fd26957ad4e4d711215692d4584be3d620d1f5c)

[romainmenke]

Changed my mind, better not to change these tests unless there is an actual issue.