Support test execution with Miri

[farnoy]

I'm using inline snapshots and INSTA_UPDATE=no. My tests otherwise work with Miri, they're only failing because of assert_snapshot!

The root cause seems to be trying to parse the Cargo manifest for ToolConfig. This is after working around auto-discovery issues with INSTA_WORKSPACE_ROOT - insta::env::ToolConfig::from_workspace

Looking at what ToolConfig does, it seems to be possible to come up with default values for cfg!(miri), like not updating the snapshots, etc.

[mitsuhiko]

I'm not sure what a good solution here is, but I'm happy to accept a patch.