Allow test-runner to filter test groups by tag

Allow test-runner to filter test groups by tag

Enable test-runner to accept a list of tags to identify
which test groups the user wishes to run.

Also allow test-runner to perform multiple iterations
of a test run.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6788