net: mctp: tests: Test that outgoing skbs have flow data populated

When CONFIG_MCTP_FLOWS is enabled, outgoing skbs should have their
SKB_EXT_MCTP extension set for drivers to consume.

Add two tests for local-to-output routing that check for the flow
extensions: one for the simple single-packet case, and one for
fragmentation.

We now make MCTP_TEST select MCTP_FLOWS, so we always get coverage of
these flow tests. The tests are skippable if MCTP_FLOWS is (otherwise)
disabled, but that would need manual config tweaking.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>