Run slow tests with flexCI

[muupan]

This PR resolves #499 by adding configs and changes to run slow tests with flexCI:

• Add flexCI configs to run slow tests
  • chainerrl.py3.cpu.slow: https://ci.preferred.jp/chainerrl.py3.cpu.slow/
  • chainerrl.py3.gpu.slow: https://ci.preferred.jp/chainerrl.py3.gpu.slow/
• Make tests/agents_tests/test_agents.py faster
• Suppress warnings due to Link.zerograds
  • Replace with Link.cleargrads and modify copy_grad function accordingly.

I confirmed that both chainerrl.py3.cpu.slow and chainerrl.py3.gpu.slow succeed when started manually:

• https://ci.preferred.jp/chainerrl.py3.cpu.slow/39542/
• https://ci.preferred.jp/chainerrl.py3.gpu.slow/39543/

After the PR is merged, I will configure the repository so that these are run by /test and required to succeed in order to merge PRs.

[muupan]

/test

[pfn-ci-bot]

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