test: get_fee and assert_fee_amount to take feerate in sat_vb

[maflcko]

Change the btc_kb argument to sat_per_vb and adjust all call sites.

This allows to remove a lossy truncation via int.

Also, the new argument should be a forced named argument.

Useful skills:

• Python 3
• The functional tests
• Fee rates in BTC/kB and sat/vB

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