Merge #17121: test: speedup wallet_backup by whitelisting peers (immediate tx relay)

581c9be0d8bff46cd68bd6a3bf72f22d11c09aea test: speedup wallet_backup by whitelisting peers (immediate tx relay) (Sebastian Falbesoner)

Pull request description:

  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.

ACKs for top commit:
  MarcoFalke:
    ACK 581c9be0d8bff46cd68bd6a3bf72f22d11c09aea, this test is testing the backup behaviour, not the tx relay behaviour
  fanquake:
    ACK 581c9be0d8bff46cd68bd6a3bf72f22d11c09aea

Tree-SHA512: d016f39cdb85501e17a74a4c4db5a9f7404baa76fbcc3675a34d3cd7bf03d7a4cb4fd3e5f17cb0597248120bb5ac8b15d3db7663007b76b010902be72954bde0

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