Fix SHOW DATA SOURCE test; must execute setup/cleanup serially

The SHOW DATA SOURCE tests were run as part of the other SHOW * tests
in test_show(), but the setup/cleanup for data sources can't be run
in parallel. This change moves the SHOW DATA SOURCE tests into a separate
test method and the setup/cleanup code is only run for this test (i.e.
not using setup_method() and teardown_method()). The test is then
only executed serially.

Change-Id: I221145f49cfe7290e132c6a87a5295b747c1fcc7
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2864
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins