tests: reduce number of generated blocks for wallet_import_rescan

Generating blocks is slow, especially when --enable-debug. There is no
need to generate a new block for each transaction, so avoid doing that
to improve this test's runtime.

test/functional/wallet_import_rescan.py

@@ -179,7 +179,16 @@ class ImportRescanTest(BitcoinTestFramework):
 
         # Create one transaction on node 0 with a unique amount for
         # each possible type of wallet import RPC.
+        last_variants = []
         for i, variant in enumerate(IMPORT_VARIANTS):
+            if i % 10 == 0:
+                blockhash = self.generate(self.nodes[0], 1)[0]
+                conf_height = self.nodes[0].getblockcount()
+                timestamp = self.nodes[0].getblockheader(blockhash)["time"]
+                for var in last_variants:
+                    var.confirmation_height = conf_height
+                    var.timestamp = timestamp
+                last_variants.clear()
             variant.label = "label {} {}".format(i, variant)
             variant.address = self.nodes[1].getaddressinfo(self.nodes[1].getnewaddress(
                 label=variant.label,
@@ -188,9 +197,15 @@ class ImportRescanTest(BitcoinTestFramework):
             variant.key = self.nodes[1].dumpprivkey(variant.address["address"])
             variant.initial_amount = get_rand_amount()
             variant.initial_txid = self.nodes[0].sendtoaddress(variant.address["address"], variant.initial_amount)
-            self.generate(self.nodes[0], 1)  # Generate one block for each send
-            variant.confirmation_height = self.nodes[0].getblockcount()
-            variant.timestamp = self.nodes[0].getblockheader(self.nodes[0].getbestblockhash())["time"]
+            last_variants.append(variant)
+
+        blockhash = self.generate(self.nodes[0], 1)[0]
+        conf_height = self.nodes[0].getblockcount()
+        timestamp = self.nodes[0].getblockheader(blockhash)["time"]
+        for var in last_variants:
+            var.confirmation_height = conf_height
+            var.timestamp = timestamp
+        last_variants.clear()
 
         # Generate a block further in the future (past the rescan window).
         assert_equal(self.nodes[0].getrawmempool(), [])
@@ -217,11 +232,14 @@ class ImportRescanTest(BitcoinTestFramework):
                 variant.check()
 
         # Create new transactions sending to each address.
-        for variant in IMPORT_VARIANTS:
+        for i, variant in enumerate(IMPORT_VARIANTS):
+            if i % 10 == 0:
+                blockhash = self.generate(self.nodes[0], 1)[0]
+                conf_height = self.nodes[0].getblockcount() + 1
             variant.sent_amount = get_rand_amount()
             variant.sent_txid = self.nodes[0].sendtoaddress(variant.address["address"], variant.sent_amount)
-            self.generate(self.nodes[0], 1)  # Generate one block for each send
-            variant.confirmation_height = self.nodes[0].getblockcount()
+            variant.confirmation_height = conf_height
+        self.generate(self.nodes[0], 1)
 
         assert_equal(self.nodes[0].getrawmempool(), [])
         self.sync_all()