This breaks up the monolithic testPaymentMgrPayment, extracting the bits
specific to testing the applyTxFees func. The goal is to make updating
the tests less cumbersome.

Two new tests are introduced, one for the core applyTxFees
functionality, and one for its error conditions.

The applyTxFees func previously took in a set of outputs as one param,
and the total value of those outputs as a separate param. This
introduced an opportunity for the function to be misued in that it could
be passed a set of outputs and a total which do not match.

An instance of this mistake actually existed in the tests for
applyTxFees. The value of the inputs and outputs was 100 DCR, but the
provided total was 5 DCR.

The test can now be run multiple times with different values so that
applyFee can be tested more thoroughly. This also enables characterizing
the existing behaviour of applyTxFees. Using absolute values in the
tests means that assertions are more precise, and any issues are more
clearly visible.

This extracts fee estimation into a func which can be reused later, as
well as fixing an issue which caused inSizes and outSizes to be twice as
long as intended, leading to fees being overestimated.

There was a couple of problems in applyTxFees which lead to fees being
greatly overestimated.

- The division to find the ratio of a single output to the total output
  amount was inverted.
- The pool fee should not be subject to paying fees, however the txfee
  was being subtracted from the total output amount rather than the
  pool fee.

A new assertion in the applyTxFees test ensures the deductions from
output amounts matches matches the expected fee to be paid.