ovn-nbctl test: Add different search string for getopt BSD variant

ovn-nbctl test: Add different search string for getopt BSD variant

2714. ovn-nbctl.at:1443: testing ovn-nbctl - commands parser error paths

fails due to:
ovn-nbctl.at:1443: ovn-nbctl --if-exists=foo list Logical_Switch
stderr:
ovn-nbctl: option '--if-exists=foo' requires an argument
./ovn-nbctl.at:1443: grep 'option .* doesn'\''t allow an argument' stderr
stdout:
./ovn-nbctl.at:1443: exit code was 1, expected 0

This is due to the difference between getopt BSD and GNU variant.

Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>