Bugfix: Add missing signet key

[moneymanolis]

Signet was missing in get_rpcconfig() resulting in such an error:

Traceback (most recent call last):
  File "/Users/manolis/.pyenv/versions/.specter/lib/python3.10/site-packages/flask/app.py", line 1523, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/manolis/.pyenv/versions/.specter/lib/python3.10/site-packages/flask/app.py", line 1509, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "/Users/manolis/.pyenv/versions/.specter/lib/python3.10/site-packages/flask_login/utils.py", line 290, in decorated_view
    return current_app.ensure_sync(func)(*args, **kwargs)
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/server_endpoints/nodes.py", line 68, in node_settings_new_node_get
    node = Node.from_json(
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/node.py", line 348, in from_json
    return cls(
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/node.py", line 327, in __init__
    self.rpc = self._get_rpc()
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/node.py", line 395, in _get_rpc
    rpc_conf_arr = autodetect_rpc_confs(
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/rpc.py", line 207, in autodetect_rpc_confs
    conf_arr.extend(_detect_rpc_confs_via_datadir(datadir=datadir))
  File "/Users/manolis/coding/repos/specter-desktop/src/cryptoadvance/specter/rpc.py", line 141, in _detect_rpc_confs_via_datadir
    if "rpcuser" in config["bitcoin.conf"][network]:
KeyError: 'signet'

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[stepansnigirev]

Test fails.

[moneymanolis]

@stepansnigirev fixed the tests.