config: no longer singleton. it is passed to Wallet.__init__

The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)

electrum/tests/test_lnhtlc.py

@@ -1,14 +1,18 @@
 from pprint import pprint
 import unittest
+from typing import NamedTuple
+
 from electrum.lnutil import RECEIVED, LOCAL, REMOTE, SENT, HTLCOwner, Direction
 from electrum.lnhtlc import HTLCManager
-from typing import NamedTuple
+
+from . import ElectrumTestCase
+
 
 class H(NamedTuple):
     owner : str
     htlc_id : int
 
-class TestHTLCManager(unittest.TestCase):
+class TestHTLCManager(ElectrumTestCase):
     def test_adding_htlcs_race(self):
         A = HTLCManager()
         B = HTLCManager()