Merge bitcoin/bitcoin#29343: test: fix wallet_import_rescan unrounded minimum amount

26ad2aeb29 test: fix wallet_import_rescan unrounded minimum amount (stickies-v)

Pull request description:

  Addresses https://github.com/bitcoin/bitcoin/pull/29283#discussion_r1468842089.

  Fixes a `JSONRPCException: Invalid amount (-3)` exception by ensuring the amount sent to `sendtoaddress` is rounded to 8 decimals.

  See https://cirrus-ci.com/task/5562947183837184?logs=ci#L2559

  Note: since `round` can also round down, `min_amount` is not _exactly_ guaranteed, but this is not a problem for the current usage. I've added a docstring to highlight this.

ACKs for top commit:
  sr-gi:
    ACK [26ad2ae](26ad2aeb29)

Tree-SHA512: 82ce16447f30535f17fa73336f7e4f74639e33215a228294b9b8005b8050a760b90a3726de279cce98c7e439f09104172b74072be3a300dbd461bf0c3f54b954

test/functional/wallet_import_rescan.py

@@ -145,8 +145,10 @@ TIMESTAMP_WINDOW = 2 * 60 * 60
 AMOUNT_DUST = 0.00000546
 
 
-def get_rand_amount():
-    r = random.uniform(AMOUNT_DUST, 1)
+def get_rand_amount(min_amount=AMOUNT_DUST):
+    assert min_amount <= 1
+    r = random.uniform(min_amount, 1)
+    # note: min_amount can get rounded down here
     return Decimal(str(round(r, 8)))
 
 
@@ -273,7 +275,7 @@ class ImportRescanTest(BitcoinTestFramework):
             variant.key = self.nodes[1].dumpprivkey(variant.address["address"])
             # Ensure output is large enough to pay for fees: conservatively assuming txsize of
             # 500 vbytes and feerate of 20 sats/vbytes
-            variant.initial_amount = max(get_rand_amount(), (500 * 20 / COIN) + AMOUNT_DUST)
+            variant.initial_amount = get_rand_amount(min_amount=((500 * 20 / COIN) + AMOUNT_DUST))
             variant.initial_txid = self.nodes[0].sendtoaddress(variant.address["address"], variant.initial_amount)
             variant.confirmation_height = 0
             variant.timestamp = timestamp