Please add "openapi-spec-validator" Python module as a GRR test package dependency

Hello !

I would like to kindly ask you to add the [`openapi-spec-validator`](https://pypi.org/project/openapi-spec-validator/) Python 3 package as a dependecy to the GRR test package.

This package is used to validate that the OpenAPI description generated for the GRR API follows the OpenAPI specification. This verification is done in the associated tests introduced by https://github.com/google/grr/pull/811.

At the moment, the tests that rely on this package are skipped if it is not installed or, if present, the module is imported inside the test function.