Description
Is your feature request related to a problem? Please describe.
I am using syrupy in a few libraries, and I find myself in a common pattern in which I write a Makefile target like make snapshot-update which invokes pytest --snapshot-update. But the real intention behind that make target is not necessarily to run all the tests and update snapshots; but to update the snapshots only.
Describe the solution you'd like
I think it'd be a neat feature to support a pytest selector which isolates only the tests depending on the snapshot fixture; so that pytest --snapshot-update could be extended to only run tests with any snapshots.
Describe alternatives you've considered
It's not a big deal! I just run all my tests when i run updates. 😄
Additional context