Add STDIN test to primer

[cooperlees]

• Check that out STDIN black support stays working
• Add asyncio.subprocess STDIN pip via communicate
• We just check we format python code from primer's lib.py

Fixes #2310

[cooperlees]

I think we need to revise the .md only file push causing no unit tests running ...

[JelleZijlstra]

Why do we need to test this with primer specifically? There are already various unit tests for stdin in test_black.py.

[cooperlees]

This ensures we don’t break the ‘-‘ cli and the whole pipeline from stdin to return value using this read from STDIN option. I've seen Unit tests miss things before and this was simple to add and fast to execute so I see no downside to this.

It also keeps our integrations tests testing as much as we can. Next I’d like to get some different CLI usages too.