config.mk: Set TEST_COVER to 0 by default (#1853)
config.mk: Set TEST_COVER to 0 by default (#1853)

Set the TEST_COVER to 0 by default so it uses optimization
-O3 when compiling.
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost