netinet6 tests: Fix Python warning

Python 3.8 warns about line 112:

'SyntaxWarning: "is" with a literal. Did you mean "=="?'

Use '==' as Python suggests.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 80430c15caac0c10832455f868fa01c912996982)