Adds the capability to test generated files

[szkiba]

The generated registry and catalog will play an important role in the k6 binary provisioning subsystem, so it is important that there is no loss of service due to generated files. It is advisable to test the generated files to see if they meet the minimum requirements.

The --test flag should be used to test the registry and catalog files generated by the --api flag. The test is successful if the file is not empty, contains k6 and at least one extension, and if all extensions meet the minimum requirements (e.g. it has versions).

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grafana/k6-ext-tooling#34