test: cover SopelMemoryWithDefault.__contains__()

Since the whole point of this type is to have a thread-safe version of
`collections.defaultdict` and not have to check for `'key' in SMWD`
before accessing something, the `__contains__()` method (supporting the
`in` keyword) is never exercised in regular code.

The rest of `SopelMemoryWithDefault` is covered by other test cases for
code that *uses* the type, so I'll just keep this patch small.

Now the only uncovered code in `sopel/tools/memories.py` is for variadic
argument handling that we plan to remove anyway, so 98% is fine for now.

test/tools/test_tools_memories.py

@@ -6,6 +6,11 @@
 from sopel.tools import identifiers, memories
 
 
+def test_sopel_default_memory_contains():
+    memory = memories.SopelMemoryWithDefault(list)
+    assert 'key' not in memory
+
+
 def test_sopel_identifier_memory_none():
     memory = memories.SopelIdentifierMemory()
     assert None not in memory