Fixed flaky MasterTest.LaunchDuplicateOfferTest.

A couple of races could occur in the "launch tasks on multiple offers"
tests where recovered resources from purposely-failed invocations turned
into a subsequent resource offer and oversaturated the expect's.

Review: https://reviews.apache.org/r/25588
Fixed the flaky FaultToleranceTest.ReconcilePendingTasks.

Review: https://reviews.apache.org/r/25604
Only perform docker validation once for tests

Review: https://reviews.apache.org/r/25569
Refactor Environment validation