Signed/Unsigned comparisons in tests

Many tests in mesos currently have comparisons between signed and unsigned integers, eg

    ASSERT_EQ(4, v1Response->read_file().size());

or comparisons between values of different enums, e.g. TaskState and v1::TaskState:

  ASSERT_EQ(TASK_STARTING, startingUpdate->status().state());

Usually, the compiler would catch these and emit a warning, but these are currently silenced because gtest headers are included using the -isystem command line flag.