partial Merge bitcoin#26722: test: speed up the two slowest functional tests by 18-35% via keypoolrefill()

BACKPORT NOTE: we have missing changes for wallet_fundrawtransaction.py due to DNM bitcoin#20536

31fdc54 test: speed up wallet_fundrawtransaction.py and wallet_sendall.py (kdmukai)

Pull request description:

  ## Problem
  `wallet_fundrawtransaction.py` and `wallet_sendall.py` are the two slowest functional tests *when running without a RAM disk*.
  ```
  # M1 MacBook Pro timings
  wallet_fundrawtransaction.py --descriptors   | ✓ Passed  | 55 s
  wallet_fundrawtransaction.py --legacy-wallet | ✓ Passed  | 381 s

  wallet_sendall.py --descriptors   | ✓ Passed  | 43 s
  wallet_sendall.py --legacy-wallet | ✓ Passed  | 327 s
  ```

  In each case, the majority of the time is spent iterating through 1500 to 1600 `getnewaddress()` calls. This is particularly slow in the `--legacy-wallet` runs.

  see:  https://github.com/bitcoin/bitcoin/blob/master/test/functional/wallet_fundrawtransaction.py#L986-L987
  see:  https://github.com/bitcoin/bitcoin/blob/master/test/functional/wallet_sendall.py#L324

  ## Solution
  Pre-fill the keypool before iterating through those `getnewaddress()` calls.

  With this change, the execution time drops to:
  ```
  wallet_fundrawtransaction.py --descriptors   | ✓ Passed  | 52 s     # -3s diff
  wallet_fundrawtransaction.py --legacy-wallet | ✓ Passed  | 291 s    # -90s diff

  wallet_sendall.py --descriptors   | ✓ Passed  | 27 s     # -16s diff
  wallet_sendall.py --legacy-wallet | ✓ Passed  | 228 s    # -99s diff
  ```

  ---

  Tagging @ Sjors as he had encouraged me to take a look at speeding up the tests.

ACKs for top commit:
  achow101:
    ACK 31fdc54

Tree-SHA512: e8dd89323551779832a407d068977c827c09dff55c1079d3c19aab39fcce6957df22b1da797ed7aa3bc2f6dd22fdf9e6f5e1a9a0200fdb16ed6042fc5f6dd992

Author: MarcoFalke

test/functional/wallet_sendall.py

@@ -322,6 +322,10 @@ def sendall_watchonly_specific_inputs(self):
     # This tests needs to be the last one otherwise @cleanup will fail with "Transaction too large" error
     def sendall_fails_with_transaction_too_large(self):
         self.log.info("Test that sendall fails if resulting transaction is too large")
+
+        # Force the wallet to bulk-generate the addresses we'll need
+        self.wallet.keypoolrefill(1600)
+
         # create many inputs
         outputs = {self.wallet.getnewaddress(): 0.000025 for _ in range(1600)}
         self.def_wallet.sendmany(amounts=outputs)