tests: add test for large payments with high gas

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What does this pull request do? Explain your changes. (required)

Adds a test to show minimum Gateway reserve when gas spikes happen.

go-livepeer uses avgGasPrice and redeemGasL2 to calculate tx cost when gas spikes happen. The result is 0.36 ETH (3 gwei * 1,200,00 * 100) will enable Gateways to not have down time when gas spikes happen.

Specific updates (required)

• Added a test to show 0.36 ETH will not fail to produce tickets.

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.93392%. Comparing base (d8e520a) to head (fa04954).

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3748         +/-   ##
===================================================
- Coverage   31.94444%   31.93392%   -0.01052%     
===================================================
  Files            158         158                 
  Lines          47520       47520                 
===================================================
- Hits           15180       15175          -5     
- Misses         31437       31441          +4     
- Partials         903         904          +1     

see 2 files with indirect coverage changes

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