Added three matchers for agent tests.
Added three matchers for agent tests.

Added a test matcher to match the `TaskID` of `Option<TaskInfo>`,
a matcher to match an `Option<TaskGroupInfo>` which contains a
task with the specified `TaskID`, and a matcher to match the `TaskID`
of `authorization::Request.Object.TaskInfo`.

Review: https://reviews.apache.org/r/66346/

gsantovena/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,gsantovena/mesos,dforsyth/mesos,kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,andschwa/mesos,gsantovena/mesos,dforsyth/mesos,asamerh4/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,andschwa/mesos,Gilbert88/mesos,abudnik/mesos,asamerh4/mesos,dforsyth/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,asamerh4/mesos,chhsia0/mesos,kaysoky/mesos,asamerh4/mesos,chhsia0/mesos,jpeach/mesos,asamerh4/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos,gsantovena/mesos,asamerh4/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,andschwa/mesos,chhsia0/mesos,gsantovena/mesos,andschwa/mesos,jpeach/mesos,Gilbert88/mesos,kaysoky/mesos