[ffigen] More test regen infra

[liamappelbe]

• Add an integration test verifying that the libclang bindings are up to date
• Migrate the JSON schema test to use the same file matching infra as everything else
• Delete generate_json_schema.dart, since it's now redundant with the UPDATE=true dart test workflow

All generated code in the FFIgen package is now covered by the same testing and updating infra.

[github-actions]

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