PY-27636: Add cwd to pythonpath's head instead of tail

current workdir must be inserted at position 0, not at
the last position because local files (like "test.py") must have priority over site-lib

python/helpers/pycharm/_jb_unittest_runner.py

@@ -31,5 +31,5 @@ if __name__ == '__main__':
     args += additional_args
     jb_doc_args("unittests", args)
     # Working dir should be on path, that is how unittest work when launched from command line
-    sys.path.append(os.getcwd())
+    sys.path.insert(0, os.getcwd())
     main(argv=args, module=None, testRunner=unittestpy.TeamcityTestRunner, buffer=not JB_DISABLE_BUFFERING)