DO NOT MERGE: tests: add some questionable but valid platform names

[lengau]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint && make test?

---

This is for discussion about what, if any, limits we want to provide for platform names.

[mr-cal]

Some thoughts:

1. Platform names are used for the container name. Craft-providers sanitizes platform names before creating a container, so this isn't a concern.
2. Platform names are used in the resulting artifact. So it may be prudent to only allow characters allowed in file paths. We could add a few extra characters and replace them. I think we are already doing this with @ or : symbols in Charmcraft.
3. For usability, we could set a maximum limit. The artifact filename should be less than 255 characters (a common max), so this could drive the maximum limit.
4. We could set a reasonable ruleset. Unless there is a real need to support the platform names in this PR, I'd be fine for something along the lines of:

Can contain alphanumerics, :, @, - and _, must start and end with an alphanumeric, max length is 64 characters.

[lengau]

I want 🇿🇦 to be a valid platform name.