Merge branch 'selftests-pmtu-Add-test-choice-and-captures'

Stefano Brivio says:

====================
selftests: pmtu: Add test choice and captures

This series adds a couple of features useful for debugging: 1/2
allows selecting single tests and 2/2 adds optional traffic
captures.

Semantics for current invocation of test script are preserved.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>