Add test case to ensure compatibility with vega-lite TopLevelSpec schema

[sramam]

An implicit assumption of using vega/ts-json-schema-generator is that we need to not add regressions to the vega-lite schemas.

When added capabilities to ts-json-schema-generator, the vega-lite constraint exposes corner cases that one is forced to add code to handle.

Due to the complexity of the Vega type-system, it is difficult and sometimes impossible to create minimal tests that exercise such code, ultimately causing degradation of test coverage and causing the CI system to fail.

An automated test would make this dependency explicit, making it easier to ensure coverage.

[domoritz]

Done in master