YAML Test CLI flag mismatch

[zhaoyuanTian123]

here is an inconsistency between the official documentation and the actual code for the YAML test CLI flag.Code: https://github.com/apache/beam/blob/master/sdks/python/apache_beam/yaml/main.py#L103 implements the flag as --test (singular),This mismatch leads to user confusion and failed test runs when following the documentation.