ZTS: Minor fixes

• The migration_012_pos.ksh test case was failing because of a missing space after log_must.

• None of the tests listed in the runfiles should include the .ksh suffix.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14515