Add basic test for TargetManager.targetSet
Add basic test for TargetManager.targetSet

Verify that if the configs change, target groups are cleaned on
TargetManager.reload (rather than having old ones linger around, even if
they are no longer present in the configs).

This covers the bug fixed in #1907 -- I verified that by checking out
source from before that commit.

This is a start on #1906

Stackdriver/stackdriver-prometheus,agaoglu/prometheus,prometheus/prometheus,amorken/prometheus,AlekSi/prometheus,Stackdriver/stackdriver-prometheus,yanghongkjxy/prometheus,tattsun/prometheus,agaoglu/prometheus,adamron1/prometheus,gottwald/prometheus,kzittritsch/prometheus,JavierZunzunegui/prometheus,Colstuwjx/prometheus,slok/prometheus,GoogleCloudPlatform/prometheus,Stackdriver/stackdriver-prometheus,SaltedMan/prometheus-annotated,brancz/prometheus,tomwilkie/prometheus,kangwoo/prometheus,Conorbro/prometheus,PrFalken/prometheus,sudhi-vm/prometheus,GoogleCloudPlatform/prometheus,kzittritsch/prometheus,dsymonds/prometheus,yanghongkjxy/prometheus,kangwoo/prometheus,sudhi-vm/prometheus,PrFalken/prometheus,kzittritsch/prometheus,PrFalken/prometheus,kangwoo/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,kzittritsch/prometheus,datajet-io/prometheus,ton31337/prometheus,amorken/prometheus,datajet-io/prometheus,alileza/prometheus,AlekSi/prometheus,SaltedMan/prometheus-annotated,slok/prometheus,Stackdriver/stackdriver-prometheus,alileza/prometheus,tomwilkie/prometheus,Conorbro/prometheus,datajet-io/prometheus,alileza/prometheus,tattsun/prometheus,agaoglu/prometheus,dsymonds/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,amorken/prometheus,Colstuwjx/prometheus,dsymonds/prometheus,keegancsmith/prometheus,sudhi-vm/prometheus,prometheus/prometheus,Colstuwjx/prometheus,GoogleCloudPlatform/prometheus,Conorbro/prometheus,prometheus/prometheus,adamron1/prometheus,brancz/prometheus,adamron1/prometheus,sudhi-vm/prometheus,AlekSi/prometheus,datajet-io/prometheus,dmilstein/prometheus,yanghongkjxy/prometheus,brancz/prometheus,slok/prometheus,ton31337/prometheus,amorken/prometheus,prometheus/prometheus,yanghongkjxy/prometheus,PrFalken/prometheus,dsymonds/prometheus,AlekSi/prometheus,Conorbro/prometheus,ton31337/prometheus,SaltedMan/prometheus-annotated,gottwald/prometheus,adamron1/prometheus,alileza/prometheus,tomwilkie/prometheus,slok/prometheus,prometheus/prometheus,ton31337/prometheus,keegancsmith/prometheus,alileza/prometheus,Colstuwjx/prometheus,dmilstein/prometheus,keegancsmith/prometheus,gottwald/prometheus,Gouthamve/prometheus,tattsun/prometheus,JavierZunzunegui/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,tattsun/prometheus,tomwilkie/prometheus,dmilstein/prometheus,gottwald/prometheus,Gouthamve/prometheus,SaltedMan/prometheus-annotated,JavierZunzunegui/prometheus,dmilstein/prometheus,Gouthamve/prometheus,brancz/prometheus,agaoglu/prometheus,JavierZunzunegui/prometheus