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/test-lib

@@ -0,0 +1,35 @@
+#!/usr/bin/env python
+
+import coverage
+import os
+import sys
+import unittest
+
+from importlib import import_module
+
+def run_all():
+    TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
+    ROOT_DIR = os.path.abspath(os.path.join(TOOLS_DIR, '..'))
+    BOTS_DIR = os.path.join(ROOT_DIR, 'zulib_bots')
+    sys.path.insert(0, BOTS_DIR)
+
+    cov = coverage.Coverage(config_file="tools/.coveragerc")
+    cov.start()
+
+    module = import_module('zulip_bots.lib_tests')
+    suite = unittest.defaultTestLoader.loadTestsFromModule(module)
+
+    suite = unittest.TestSuite([suite])
+    runner = unittest.TextTestRunner(verbosity=2)
+    result = runner.run(suite)
+    if result.failures or result.errors:
+        sys.exit(1)
+
+    cov.stop()
+    cov.data_suffix = False  # Disable suffix so that filename is .coverage
+    cov.save()
+    cov.html_report()
+    print("HTML report saved under directory 'htmlcov'.")
+
+if __name__ == '__main__':
+    run_all()