Merge bitcoin/bitcoin#30714: test: fix `TestShell` initialization (late follow-up for #30463)

bd7ce05f9d test: fix `TestShell` initialization (late follow-up for #30463) (Sebastian Falbesoner)

Pull request description:

  Creating a `TestShell` instance as stated in the [docs](https://github.com/bitcoin/bitcoin/blob/master/test/functional/test-shell.md) currently fails on master:
  ```
  $ python3
  Python 3.10.13 (main, Mar 15 2024, 07:36:23) [Clang 16.0.6 ] on openbsd7
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import sys
  >>> sys.path.insert(0, "/home/thestack/bitcoin/test/functional")
  >>> from test_framework.test_shell import TestShell
  >>> test = TestShell().setup(num_nodes=2, setup_clean_chain=True)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/home/thestack/bitcoin/test/functional/test_framework/test_shell.py", line 70, in __new__
      TestShell.instance = TestShell.__TestShell()
  TypeError: BitcoinTestFramework.__init__() missing 1 required positional argument: 'test_file'
  ```
  Since #30463, BitcoinTestFramework instances expect the path of the calling test at construction, in order to find shared data like the configuration (config.ini) and the cache. Note that in contrast to actual functional tests, we can't simply pass `__file__` here, as the test shell module sits within the `test_framework` subfolder, so we have to navigate up to the parent directory and append some dummy test file name.

  On the long-term we should probably add some TestShell instantation smoke-test to detect issues like this early. As I'm not too familiar with the CI I'm not sure what is a good way to achieve this (a functional test obviously can't be used, as that's already a BitcoinTestFramework test in itself), but happy to take suggestions.

ACKs for top commit:
  ismaelsadeeq:
    Tested ACK bd7ce05f9d
  danielabrozzoni:
    tACK bd7ce05f9d
  brunoerg:
    ACK bd7ce05f9d

Tree-SHA512: c3a2365e2cda48a233ee724673c490787981354914f33e10eadbbad9c68e8403d84c5551229a611401e743886539de380ba4bfcb77032b6c85731e3bbe962dc1

test/functional/test_framework/test_shell.py

@@ -2,9 +2,11 @@
 # Copyright (c) 2019-2022 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
+import pathlib
 
 from test_framework.test_framework import BitcoinTestFramework
 
+
 class TestShell:
     """Wrapper Class for BitcoinTestFramework.
 
@@ -67,7 +69,13 @@ class TestShell:
         # This implementation enforces singleton pattern, and will return the
         # previously initialized instance if available
         if not TestShell.instance:
-            TestShell.instance = TestShell.__TestShell()
+            # BitcoinTestFramework instances are supposed to be constructed with the path
+            # of the calling test in order to find shared data like configuration and the
+            # cache. Since TestShell is meant for interactive use, there is no concrete
+            # test; passing a dummy name is fine though, as only the containing directory
+            # is relevant for successful initialization.
+            tests_directory = pathlib.Path(__file__).resolve().parent.parent
+            TestShell.instance = TestShell.__TestShell(tests_directory / "testshell_dummy.py")
             TestShell.instance.running = False
         return TestShell.instance