[tests] make one scenario of client-test more stable

This patch makes ClientTest.TestScannerKeepAlive more stable.

The motivation for this patch was seeing the following test
failure in a TSAN build [1]:

  src/kudu/client/client-test.cc:2763
  Failed
  Bad status: Not found: Scanner 0d9072070e4443ba8c49b089806415b4 not found (it may have expired)

Before: 2 out of 32 failed
  http://dist-test.cloudera.org/job?job_id=aserbin.1649379879.23423

After:  0 out of 32 failed
  http://dist-test.cloudera.org/job?job_id=aserbin.1649381490.37476

[1] http://jenkins.kudu.apache.org/job/kudu-gerrit/25299/BUILD_TYPE=TSAN

Change-Id: Ia7ebddc5499d59e37282e4f1e6513bc5f83cdb8a
Reviewed-on: http://gerrit.cloudera.org:8080/18396
Reviewed-by: Yingchun Lai <acelyc1112009@gmail.com>
Tested-by: Alexey Serbin <alexey@apache.org>
1 file changed