Scheduled Txs: Update FlowTransactionScheduler tests to be less flaky

[joshuahannan]

Issue To Be Solved

The tests for FlowTransactionScheduler fail a significant portion of the time because the timestamp in the testing framework is not reliable. We need to update the tests in some way to make that less likely to happen.

Suggest A Solution

• Look into testing framework updates to see if there is a way to fix the problem at the root and make the timestamp more reliable
• potentially use larger intervals for timestamp manipulation
• Potentially schedule transactions in a way that allows for larger intervals
• The errors are almost always in the schedule_test.cdc when we check for the pending queue, so we could update the assertions to be a little more forgiving if the timestamp has unintentionally drifted

Additional Test Updates

• Avoid hardcoded values like fee refund ratio in tests: FEATURE: Transaction scheduling #489 (comment)
• Make the cancel limits tests more clear about their parameters