[rllib] Improve test learning check, fix flaky two step qmix

[krfricke]

Why are these changes needed?

two step qmix was previously very flaky.

The core test has been fixed by providing a seed. For this to work, passing the seed in a GroupAgentWrapper had to be enabled.

Secondly, learning test checks recently used analysis.trials[0].last_result to check for success. This PR introduces a change, that collects all trials results and picks the best achieved reward. This means that the test will always succeed if at least one of the trials achieved the desired performance.

• This won't make a difference for single trial runs.
• This is usually good for parameter sweeps, as we expect at least one trial to achieve good performance, not all
• This might not be suitable for some tests that require that all trials achieve good performance. However, since the test was previously just about testing that the first trial achieved good performance, this doesn't seem like a regression to me.

cc @ericl, @sven1977 and @richardliaw for your opinion on this change.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[richardliaw]

This change looks fine to me.

[krfricke]

I'll merge because I don't see how this should lead to any regressions. We can revert if someone has concerns.