Stabilize and speed up A3C tests
Stabilize and speed up A3C tests

Before this commit:
- test_a3c.py succeeds only 4 times out of 10
- when suceeding, it takes 90-120 seconds

After this commit:
- it suceeds 10/10
- it takes 23-28 seconds

toslunar/chainerrl,toslunar/chainerrl