Updated the gtest filter in Environment constructor rather than in
'SetUp' in order to make sure the filter is properly set before
invoking RUN_ALL_TESTS.

Review: https://reviews.apache.org/r/9868

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455413 13f79535-47bb-0310-9956-ffa450edef68