Config: Add more utest for env config. v6.0.147 v7.0.4

[suzp1984]

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

https://github.com/ossrs/srs/blob/16e569d82357757ddac6ef91d7a5fe7837319909/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.