Fix failing op/magic.t test 157

Change 888a67f6b9 added manipulation of __NoNeSuCh2, but it gets
accidentally picked up by the 'set' command looking for __NoNeSuCh in test
157 because 'set' will display the value of *all* variables whose prefix
matches the name given to the command.
Silence irrelevant stderr output from op/magic.t

Some 'set' commands in this test are intended to verify that an environment
variable is not set, which causes the command to send a warning message to
stderr.