selftests: Include functional test for journal plugin.

Include a functional test for the journal plugin that
validates the database file created by the test runner.

It basically validates the job_id inside the job_info table
and the number of rows inside the test_journal table,
for a simple execution of sleeptest.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
selftests: split tests in journal plugin functional test.

Split test in two, for the functional journal plugin tests.
Using a test fixture to setup (to execute and catch JSON and journal)
and tear down (to close the database) things.
Merge pull request #205 from ruda/journal_plugin_functional_tests

selftests: Include functional test for journal plugin.