Config: Add more utest for env config. v6.0.147 (#4142)

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 #4092

16e569d823/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>

trunk/src/app/srs_app_config.hpp

@@ -305,6 +305,9 @@ private:
 protected:
     // The directive root.
     SrsConfDirective* root;
+private:
+    // The cache for parsing the config from environment variables.
+    SrsConfDirective* env_cache_;
 // Reload  section
 private:
     // The reload subscribers, when reload, callback all handlers.