MFC r339836, r340286, r341358:

pf tests: Test ':0' ignoring link-local addresses

Fix test: sys.netpfil.pf.pass_block.noalias

Replace hard-coded epair0b with the variable holds the real epair interface
used for testing.

pf tests: Make pass_block:noalias more robust

Send several ICMPv6 echo requests. We've seen occasional failures with a
single request.

PR: 201695