Add unit test for Inverse in pkg/labels/matcher.go (#6586)
Add unit test for Inverse in pkg/labels/matcher.go (#6586)

This PR is about adding a unit test for Inverse and updating the other test in pkg/labels/matcher.go.

Signed-off-by: Hu Shuai <a4797ccdc3c71d8ffc5c17a5bb7f114e3818f278@cn.fujitsu.com>
prometheus/prometheus,Gouthamve/prometheus,alileza/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,alileza/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,Gouthamve/prometheus,prometheus/prometheus,alileza/prometheus,kangwoo/prometheus,Gouthamve/prometheus,kangwoo/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus