config: make sure running the unittests can never mutate prod config

[SomberNight]

related: #10883

Running the unittests must never mutate the mainnet/production config file (or other files in the datadir) of a dev machine. I am not sure how exactly to prevent that, but this is one approach.

The idea is that a test case constructing e.g. a config file should use a tempfolder for it (and then clean-up after itself), however in case that is accidentally not done, we should have a belt-and-suspenders last-resort protection. In this PR I propose using e.g. /home/user/.electrum/unittests/config instead of /home/user/.electrum/config: we always know we are in a unittest context, and if so, we add an extra level of nesting.

[f321x]

Might be nice if regtest tests also have some fallback, the unixsocket test for example writes into the ~/.electrum/regtest dir.

This might also work and gets passed on to the regtests without requiring modifications to SimpleConfig or writing in the user dir at all. Otoh setting an env variable might be more fragile?

diff --git a/tests/__init__.py b/tests/__init__.py
index 1dc6c680ae..dc64d4228d 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -1,4 +1,4 @@
-import asyncio
+import atexit
 import os
 import unittest
 import threading
@@ -30,7 +30,8 @@ FAST_TESTS = False
 electrum.logging._configure_stderr_logging(verbosity="*")
 
 electrum.util.AS_LIB_USER_I_WANT_TO_MANAGE_MY_OWN_ASYNCIO_LOOP = True
-electrum.simple_config._RUNNING_UNITTESTS = True
+os.environ["ELECTRUMDIR"] = _unittests_datadir = tempfile.mkdtemp(prefix="electrum-unittests-datadir-")
+atexit.register(lambda: shutil.rmtree(_unittests_datadir, ignore_errors=True))
 
 
 class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):

[ecdsa]

I do not like the multiplication of paths.
#10833 introduced tempfile.mkdtemp(prefix="electrum-unittest-base-")
Why not assert here that electrum_path has been set and is starting with that prefix?