* getproviders ParsePlatform: add check for invalid platform strings with too many parts
The existing logic would not catch things like a platform string containing multiple underscores. I've added an explicit check for exactly 2 parts and some basic tests to prove it.
* command/providers-lock: add tests
This commit adds some simple tests for the providers lock command. While adding this test I noticed that there was a mis-copied error message, so I replaced that with a more specific message. I also added .terraform.lock.hcl to our gitignore for hopefully obvious reasons.
getproviders.ParsePlatform: use parts in place of slice range, since it's available
* command: Providers mirror tests
The providers mirror command is already well tested in e2e tests, so this includes only the most absolutely basic test case.
