Clarify or improve globbing support for `fga model test`

[ewanharris]

The glob support for the --tests option in fga model test uses the Go filepath.Glob, which uses filepath.Match under the hood to match the glob pattern.

The means that we're missing support for ** in the glob pattern (golang/go#11862) as the Go API is intended to mirror the C std lib rather than the bash globbing that some might expect.

** is especially useful as it would allow matching any number of path segments meaning that in a structure like in the test fixtures:

• ls ./tests/fixtures/**/*.fga.yaml resolves both the basic-store.fga.yaml and relative-path/relative-path-store.fga.yaml
• fga model test --tests "./tests/fixtures/**/*.fga.yaml" will only resolve relative-path-store.fga.yaml

Given that it might be expected for the globbing to match the behaviour of bash, this might lead to folks accidentally not running their entire tests if they use a tool like ls to help determine the globbing pattern used.

We should either:

• Document this limitation
• Improve the globbing to match what is most likely expected