Add tools/test-lib.

This starts to add direct test coverage on
zulip_bots/zulip_bots/lib.py.

It is not yet integrated into tools/test-main.

tools/run-mypy

@@ -28,6 +28,8 @@ exclude = [
     # Excluded out of laziness:
     "zulip_bots/zulip_bots/terminal.py",
     "zulip_bots/zulip_bots/simple_lib.py",
+    "zulip_bots/zulip_bots/lib_tests.py",
+    "tools/test-lib",
 ]
 
 parser = argparse.ArgumentParser(description="Run mypy on files tracked by git.")