Deprecate "environment: staging"

[jku]

The test suite currently uses/supports the staging environment in a few ways:

• the client-under-test CLI defines --staging as something the client should support
• the action takes environment as argument: originally the idea was to run more tests on whatever environment was specified -- but later the addition of --trusted-root and --signing-config meant that there are now more economic ways to develop the suite than duplicating test assets: so currently there is only a single test test_simple that gets run when environment: staging, this test only ensures that the client can successfully sign and verify with the given environment
• loads of verification tests in practice use bundles from staging environment, but these tests use --trusted-root and don't require either --staging or environment: staging

Plan:

• Keep --staging in the client-under-test CLI: we do want to encourage clients to include a feature like that
• Deprecate environment argument
• Duplicate the test_simple() test that current depends on environment so that it tests production and staging in the same test run. This requires refactoring the client() fixture a bit but it should be a simplification