Improve test_simple.py and check discrete actions

[chriselion]

• Use pytest.mark.parametrize to run multiple variants of the same basic test
• Pick a random goal each "run" and pass that as an observation (instead of always going to +1)
• Add a variant that covers discrete actions

[ervteng]

I think we should move the actual running of the test to the test_ppo.py file - what do you think?

[chriselion]

Right now it's only testing PPO, but I think it's general enough to test other combinations of settings (memory, visual obs) and other trainers (SAC).

[xiaomaogy]

I guess it is testing the rl aspect of the environment, so either rename it something like test_simple_rl or move it to test_ppo? The PR looks good to me, it is ok to merge.

[chriselion]

Sounds good, I renamed to test_simple_rl.py and moved it to the main trainers/tests directory.