Fix zfs-tests.sh single test functionality

Without any tag specified into the runtime-generated runfile the
test-runner will not execute the test provided from the command line:
fix this by adding tag information to the custom runfile.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6826