Added a test for #918

[hackaugusto]

Description

Issue #918 needs that the reveal timeout and settle timeout are validated against each other.

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Quality check list that cannot be automatically verified.

• Safety
  • The changes respect the necessary conditions for safety (https://raiden-network-specification.readthedocs.io/en/latest/smart_contracts.html#protocol-values-constraints)
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[codecov]

Codecov Report

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  Misses             ?     2123           
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Impacted Files	Coverage Δ
raiden/api/python.py	68.33% <ø> (ø)

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[karlb]

Potential additions:

• explanation: why is this settle timeout too small
• test that it does not raise an exception for larger settle timeouts, making sure that the threshold is exactly where it should be

[karlb]

Actually, I was suggesting to test that channel_open will succeed for settle_timeout = node1.raiden.config["reveal_timeout"] * 2 to verify that the settle_timeout validity threshold is honored in both directions.

But maybe the condition is so simple that a test for that is overkill. Add it or leave it at your own discretion.

[hackaugusto]

done