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

srs/trunk/src/app/srs_app_config.cpp

Lines 71 to 82 in 16e569d

	#define SRS_OVERWRITE_BY_ENV_DIRECTIVE(key) { \
	static SrsConfDirective* dir = NULL; \
	if (!dir && !srs_getenv(key).empty()) { \
	std::vector<string> vec = srs_string_split(srs_getenv(key), " "); \
	dir = new SrsConfDirective(); \
	dir->name = key; \
	for (size_t i = 0; i < vec.size(); ++i) { \
	dir->args.push_back(vec[i]); \
	} \
	} \
	if (dir) return dir; \
	}

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.