Use DEFAULT_MEDIATION_FEE_MARGIN in test

[karlb]

Description

in test_mediated_transfer_with_fees. In its current form, the test
works without this change. But changing the fee amount in the test or
the DEFAULT_MEDIATION_FEE_MARGIN will break the test without clearly
showing the cause. So it's better to handle this case explicitly.

PR review check list

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)
• Code quality
  • Error conditions are handled
  • Exceptions are propagated to the correct parent greenlet
  • Exceptions are correctly classified as recoverable or unrecoverable
• Compatibility
  • State changes are forward compatible
  • Transport messages are backwards and forward compatible
• Commits
  • Have good messages
  • Squashed unecessary commits
• If it's a bug fix:
  • Regression test for the bug
    • Properly covers the bug
    • If an integration test is used, it could not be written as a unit test
• Documentation
  • A new CLI flag was introduced, is there documentation that explains usage?
• Specs
  • If this is a protocol change, are the specs updated accordingly? If so, please link PR from the specs repo.
• Is it a user facing feature/bug fix?
  • Changelog entry has been added

[codecov]

Codecov Report

Merging #4786 into develop will decrease coverage by 0.13%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4786      +/-   ##
===========================================
- Coverage    80.92%   80.79%   -0.14%     
===========================================
  Files          119      118       -1     
  Lines        14343    14332      -11     
  Branches      2209     2207       -2     
===========================================
- Hits         11607    11579      -28     
- Misses        2087     2098      +11     
- Partials       649      655       +6

Impacted Files	Coverage Δ
raiden/storage/restore.py	88.88% <0%> (-4.77%)	⬇️
raiden/storage/serialization/types.py	64.17% <0%> (-4.48%)	⬇️
raiden/waiting.py	81.54% <0%> (-4.17%)	⬇️
raiden/network/transport/matrix/client.py	68.5% <0%> (-1.3%)	⬇️
raiden/ui/app.py	24.6% <0%> (-0.8%)	⬇️
raiden/api/python.py	68.25% <0%> (-0.53%)	⬇️
raiden/network/transport/matrix/transport.py	80.74% <0%> (-0.3%)	⬇️
raiden/network/proxies/token_network.py	51.45% <0%> (-0.13%)	⬇️
raiden/utils/mediation_fees.py

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[palango]

Thanks!