discovery/marathon: Fix race conditions in test
discovery/marathon: Fix race conditions in test

The concurrency applied before is in most cases not even needed. With
a cap=1 channel, most tests are much cleaner.

TestMarathonSDRunAndStop was trickier. It could even have blocked
before.

This also includes a general refactoring of the whole file.

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