[core] Fix ray.timeline()

[stephanie-wang]

Why are these changes needed?

Fix a typo bug in ray.timeline() and add a basic test.

[stephanie-wang]

@rickyyx I couldn't figure out how to test the actual content. It looks like this is using a different codepath from the chrome_tracing_dump that's tested in the rest of the file? ray._private.profiling.chrome_tracing_dump vs GlobalState.chrome_tracing_dump. I left a TODO to test this / squash into one codepath?

[rickyyx]

@rickyyx I couldn't figure out how to test the actual content. It looks like this is using a different codepath from the chrome_tracing_dump that's tested in the rest of the file? ray._private.profiling.chrome_tracing_dump vs GlobalState.chrome_tracing_dump. I left a TODO to test this / squash into one codepath?

Yeah - I think it was a miss that this wasn't tested when we changed the profiling feature last time. I can stamp this for now while I cleaned up the migration.