test_helpers: Fix a nonexistent import.

This module doesn't exist, and never did; the name appears to be a
mistaken variant of the module that really does contain ZulipTestCase.
So, fix the import to use the real name.

This would never have worked at runtime, which is why it's in an
`if False:`.  It's also an example of the kind of error that can be
hidden by `ignore_missing_imports`; we'd have caught the issue
immediately if we hadn't had a blanket application of that flag
in place.

zerver/lib/test_helpers.py

@@ -46,9 +46,7 @@ from zerver.models import (
 )
 
 from zerver.lib.request import JsonableError
-
-if False:
-    from zerver.lib.test_case import ZulipTestCase
+from zerver.lib.test_classes import ZulipTestCase
 
 import collections
 import base64
@@ -291,7 +289,7 @@ def instrument_url(f: UrlFuncT) -> UrlFuncT:
     if not INSTRUMENTING:  # nocoverage -- option is always enabled; should we remove?
         return f
     else:
-        def wrapper(self: 'ZulipTestCase', url: str, info: Dict[str, Any]={},
+        def wrapper(self: ZulipTestCase, url: str, info: Dict[str, Any]={},
                     **kwargs: Any) -> HttpResponse:
             start = time.time()
             result = f(self, url, info, **kwargs)