[tests] fix flakiness in TestSpaceAvailableMetrics

I saw TabletServerDiskErrorITest.TestSpaceAvailableMetrics failing
with an error like below in a pre-commit test run [1] (TSAN build):

  src/kudu/integration-tests/disk_failure-itest.cc:352: Failure
  Expected equality of these values:
    wal_dir_space
      Which is: 40835236209
    wal_dir_space_refetch
      Which is: 40767713649

This patch addresses the issue.

[1] http://dist-test.cloudera.org/job?job_id=jenkins-slave.1703028347.1027630

Change-Id: Iec10ca3d19aa5e378cebd3ceef6368496797229f
Reviewed-on: http://gerrit.cloudera.org:8080/20821
Tested-by: Alexey Serbin <alexey@apache.org>
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
1 file changed