Fixed two flaky tests in GarbageCollectorIntegrationTest.

Review: https://reviews.apache.org/r/24350
Added a trailing space to _CheckFatal's output.

- So there is a space between the error message and the additional message appended after the check. e.g. CHECK_SOME(None()) << "Some additional message";
- GLOG CHECK adds this space too: https://code.google.com/p/google-glog/source/browse/trunk/src/glog/logging.h.in#561

Review: https://reviews.apache.org/r/24355
Fixed docker validation and only run docker tests on linux with root.

Review: https://reviews.apache.org/r/24361