Improve Megatron-RL testing

The native RL training loop in Megatron-RL is vulnerable to breaking changes due to insufficient test coverage. This issue is for tracking tests we wish to add:

- [ ] unit test to trigger alert when default params for RL training are changed (in args or hardcoded, e.g. in inference)
- [ ] functional test with high memory pressure model to alert to changes in memory consumption
- [ ] add functional tests for the sequence parallelism plus sequence packing case