Schema version 1.19 introduced two changes:
1. authMechanism field on runOnRequirement: tests must be skipped unless the server supports the specified auth mechanism.
2. Fields in uriOptions may be $$placeholder documents; the runner must replace them with values loaded from the test environment (e.g. for OIDC: authMechanismProperties: {ENVIRONMENT: {$$placeholder: 1}}).

Spec: https://github.com/mongodb/specifications/blob/master/source/unified-test-format/unified-test-format.md