Make threadpool-test less flaky
Make threadpool-test less flaky

Under heavy load, threadpool-test may take more than 10 seconds in rare
cases and cause a test failure. This change increases the timeout.

Also reliably disable the alarm, even if an assertion fires, using a
ScopedCleanup object.

Change-Id: I6465922e5b43dc9aff80fa2b6e6549061bfd0425
Reviewed-on: http://gerrit.cloudera.org:8080/5293
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Tested-by: Kudu Jenkins

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