test-backend: Cast test runner to the custom Runner type.

Mypy does not know the acccurate return type of `get_runner` that is
determined by the `TEST_RUNNER` setting. We need to cast it to the
correct type to use methods like `get_shallow_tested_templates`. Note
that we import conditionally to avoid adding runtime dependency on
`zerver`.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

tools/test-backend

@@ -8,9 +8,14 @@ import shlex
 import subprocess
 import sys
 import tempfile
-from typing import Iterator, List
+from typing import TYPE_CHECKING, Iterator, List, Type, cast
 from unittest import mock
 
+if TYPE_CHECKING:
+    # This script does not have access to the zerver module during runtime.
+    # We can only import this when type checking.
+    from zerver.lib.test_runner import Runner
+
 TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
 os.chdir(os.path.dirname(TOOLS_DIR))
 sys.path.insert(0, os.path.dirname(TOOLS_DIR))
@@ -406,7 +411,9 @@ def main() -> None:
 
     subprocess.check_call(["tools/webpack", "--test"])
 
-    TestRunner = get_runner(settings)
+    # isinstance check cannot be used with types. This can potentially improved by supporting
+    # dynamic resolution of the test runner type with the django-stubs mypy plugin.
+    TestRunner = cast("Type[Runner]", get_runner(settings))
 
     parallel = default_parallel if options.processes is None else options.processes
     if parallel > 1: