tablet_server-test: wait for leader in some tests
tablet_server-test: wait for leader in some tests

The fix for KUDU-2463 made successful Raft elections a prerequisite for
scanning empty tablets. Under enough stress, even in a single replica
case, this may not complete immediately, and can thus, lead scans to
return a different error than expected. This patch updates the test
fixture that creates an empty tablet in tablet_server-test to wait until
this has passed before deeming the tablet ready for business.

The following test incantation:
tablet_server-test --gtest_filter=*BadProjectionTypes* --stress_cpu_threads=100

would fail fairly consistently without this patch.

Change-Id: Ie6ca079353d4d44a48defc386d94b4bda452ec1b
Reviewed-on: http://gerrit.cloudera.org:8080/11718
Tested-by: Andrew Wong <b68e4fdc6430321a6b47400732ff97d7ae91234e@cloudera.com>
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

InspurUSA/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu