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")

tests/sys/netinet6/frag6/frag6_16.py

@@ -109,7 +109,7 @@ def main():
 	foffset=(int)(1288/8)
 	mbit=1
 	for i in range(1,30):
-		if i is 29:
+		if i == 29:
 			mbit=0
 		ip6f0n = sp.Ether() / \
 			sp.IPv6(src=args.src[0], dst=args.to[0]) / \