tests: add "as_testnet" fn decorator, to run single tests in testnet

As opposed to using TestCaseForTestnet class, this allows having a single class
of many related unit tests, some using testnet and some using mainnet constants.

electrum/tests/__init__.py

@@ -51,6 +51,7 @@ class ElectrumTestCase(SequentialTestCase):
 
 
 class TestCaseForTestnet(ElectrumTestCase):
+    """Class that runs member tests in testnet mode"""
 
     @classmethod
     def setUpClass(cls):
@@ -61,3 +62,19 @@ class TestCaseForTestnet(ElectrumTestCase):
     def tearDownClass(cls):
         super().tearDownClass()
         constants.set_mainnet()
+
+
+def as_testnet(func):
+    """Function decorator to run a single unit test in testnet mode.
+
+    NOTE: this is inherently sequential; tests running in parallel would break things
+    """
+    def run_test(*args, **kwargs):
+        old_net = constants.net
+        try:
+            constants.set_testnet()
+            func(*args, **kwargs)
+        finally:
+            constants.net = old_net
+    return run_test
+