Some test-running changes

[Todd Lipcon <to...@cloudera.com>]

Hey Kudu devs,

I just committed https://gerrit.cloudera.org/#/c/9470/ which made a few
changes to how tests are run. See the commit message for details, but
overview of the things that might affect you:

- if you often loop an entire suite (eg raft_consensus-itest) using
dist_test.py loop, you probably now want to do so like this:

build-support/dist_test.py run -n 100 -R raft_consensus-itest

... in order to preserve the sharding

If you still want to loop just a single test case, the old syntax should be
fine:

build-support/dist_test.py loop -n 100 --
build/latest/bin/raft_consensus-itest --gtest_filter=Foo

Also, if you use 'ctest' to run tests, you'll notice that they are now
sharded which should make them run a bit faster and avoid timeouts.

Feel free to ping me if you hit any issues with precommits or dev workflows.

-Todd
-- 
Todd Lipcon
Software Engineer, Cloudera