test: speedup wallet_backup by whitelisting peers (immediate tx relay)

approaches part of #16613 ("Functional test suite bottlenecks")

The majority of the test time is spent in sync_mempools() after sending to
addresses, i.e. the bottleneck is in relaying transactions. By whitelisting the
peers via -whitelist, the inventory is transmissioned immediately rather than
on average every 5 seconds, speeding up the test by at least a factor of two:

before:
$ time ./wallet_backup.py
real    2m2.523s
user    0m6.093s
sys 0m2.454s

with this PR:
$ time ./wallet_backup_with_whitelist.py
real    0m36.570s
user    0m5.365s
sys 0m1.696s

Note that the test is not deterministic (the sendtoaddress RPC in function
one_send() is executed with a probability of 50%), hence the times could vary
between individual runs.

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