1. don't use static variable to store the result;
2. add more UT to handle the multi value and values with whitespaces;
related to #409216e569d823/trunk/src/app/srs_app_config.cpp (L71-L82)
`static SrsConfDirective* dir` removed, this static var here is to avoid
the memory leak, I add the `SrsConfDirective` instance to the `env_dirs`
directive container, which will destroy itself inside `SrsConfig`
destructor.
---------
Co-authored-by: winlin <winlinvip@gmail.com>
133a39a81d