add rules for tests/ in CODEOWNERS (#3226)
add rules for tests/ in CODEOWNERS (#3226)


henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM