Merge bitcoin/bitcoin#27884: test: Use TestNode datadir_path or chain_path where possible

aaaa3aefbd test: Use TestNode *_path properties where possible (MarcoFalke)
dddd89962b test: Allow pathlib.Path as RPC argument via authproxy (MarcoFalke)
fa41614a0a scripted-diff: Use wallets_path and chain_path where possible (MarcoFalke)
fa493fadfb test: Use wallet_dir lambda in wallet_multiwallet test where possible (MarcoFalke)

Pull request description:

  It seems inconsistent, fragile and verbose to:

  * Call `get_datadir_path` to recreate the path that already exists as field in TestNode
  * Call `os.path.join` with the hardcoded chain name or `self.chain` to recreate the TestNode `chain_path` property
  * Sometimes even use the hardcoded node dir name (`"node0"`)

  Fix all issues by using the TestNode properties.

ACKs for top commit:
  willcl-ark:
    re-ACK aaaa3aefbd
  theStack:
    Code-review ACK aaaa3aefbd 🌊

Tree-SHA512: e4720278085beb8164e1fe6c1aa18f601558a9263494ce69a83764c1487007de63ebb51d1b1151862dc4d5b49ded6162a5c1553cd30ea1c28627d447db4d8e72

test/functional/feature_startupnotify.py

@@ -3,9 +3,6 @@
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test -startupnotify."""
-
-import os
-
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
@@ -20,12 +17,12 @@ class StartupNotifyTest(BitcoinTestFramework):
         self.num_nodes = 1
 
     def run_test(self):
-        tmpdir_file = os.path.join(self.options.tmpdir, NODE_DIR, FILE_NAME)
-        assert not os.path.exists(tmpdir_file)
+        tmpdir_file = self.nodes[0].datadir_path / FILE_NAME
+        assert not tmpdir_file.exists()
 
         self.log.info("Test -startupnotify command is run when node starts")
         self.restart_node(0, extra_args=[f"-startupnotify=echo '{FILE_NAME}' >> {NODE_DIR}/{FILE_NAME}"])
-        self.wait_until(lambda: os.path.exists(tmpdir_file))
+        self.wait_until(lambda: tmpdir_file.exists())
 
         self.log.info("Test -startupnotify is executed once")