(rpc-test) accurately account for fee without rounding error
Fix for #2807, this test compares balances after doing key exports and imports, and expects these balances to be equal. But they are not exactly equal due to transaction fees, so the test makes them "equal" by rounding a value that has had fees taken out up to the nearest hundredth of a unit (which is much more than the default fee). This obviously is somewhat sloppy. It also converts a balance to float, which really should never be used due to loss of precision (use Decimal instead).
This change makes the test accurately account for the fee using precise comparisons, and removes the use of float. This test doesn't depend on the default fee (0.0001) but instead sets the fee (to that value). This way, if the default fee changes in the future, this test will continue to run. While testing these changes, I set the fee to various values (up to the max, 0.0190), and the test still passes.
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