ZTS: replace su commands by run_user function

ZTS: replace su commands by run_user function

Needed for PATH variable to be passed into su.  The
posix* tests were fixed, but they need further investigation
before they can be enabled.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #6303