tests: add unittest for Abstract_Wallet.export_history_to_file

Adds unittest for Abstract_Wallet.export_history_to_file that
compares the output against reference files. This
should help to prevent regressions and ensure the layout
of the export stays static over time.

tests/__init__.py

@@ -35,9 +35,10 @@ electrum.util.AS_LIB_USER_I_WANT_TO_MANAGE_MY_OWN_ASYNCIO_LOOP = True
 class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
     """Base class for our unit tests."""
 
-    TESTNET = False
+    TESTNET = False  # there is also an @as_testnet decorator to run single tests in testnet mode
     REGTEST = False
     TEST_ANCHOR_CHANNELS = False
+    WALLET_FILES_DIR = os.path.join(os.path.dirname(__file__), "test_storage_upgrade")
     # maxDiff = None  # for debugging
 
     # some unit tests are modifying globals... so we run sequentially:
@@ -110,6 +111,9 @@ class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
         self._lnworkers_created.append(lnwallet)
         return lnwallet
 
+    def get_wallet_file_path(self, wallet_name: str) -> str:
+        return os.path.join(self.WALLET_FILES_DIR, wallet_name)
+
 
 def as_testnet(func):
     """Function decorator to run a single unit test in testnet mode.