Fix a warning in Perl test code

Fix a warning in Perl test code

The code was passing a scalar argument to node->restart(), but it was
expecting a hash, which causes a warning from Perl ("Odd number of
elements in hash assignment").

But the node->restart() function doesn't take a mode argument anyway.
This was probably copied from an incorrect comment (see commit
750c59d7ec).  The default restart mode is already "fast", so the test
should still be semantically correct without explicitly specifying the
mode.

Discussion: https://www.postgresql.org/message-id/e3f4bf1b-63d3-408a-b07e-d35a0fdf1b98@eisentraut.org