test: Add cost_of_change parameter assertions to bnb_search_test
Backport of core PR18195.
Since we don't support multiplication with a double (see amount.h),
the 0.9 * CENT has been updated to 9 * CENT / 10 and so on, so the
amounts remain integers.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8353