glob!: Add three-arg version to specify base_path

[the-kenny]

The new three-arg glob! allows users to specify the base directory the glob is working against. This works around the limitation that blobs like ../foo/*.json do not resolve with the current walkdir + globset implementation.

This allows the usage of glob! in nested situations where the globbed files are in a parent directory:

src
├── main.rs
├── snapshots
│   └── insta_glob_test__insta_glob_dir.snap
├── submod
│   └── mod.rs -- glob! tests are here
└── test_data
    └── foo.json

glob!("../test_data", "*.json", |path| { ... })

[the-kenny]

Note that this implementation could provide a nicer interface where glob! or glob_exec would accept a glob like ../test_data/*.json. However, implementation of this isn't as easy as I hoped as we would need to detect & split the path elements from the glob elements (i.E. ../test_data/*.json -> ("../test_data/", "*.json")).

I can tackle this if you prefer this simpler interface.

[mitsuhiko]

This looks good to me. Thank you!