selftests: forwarding: add tunnel_key "nofrag" test case

Add a selftest that configures metadata tunnel encapsulation using the TC
"tunnel_key" action: it includes a test case for setting "nofrag" flag.

Example output:

 # selftests: net/forwarding: tc_tunnel_key.sh
 # TEST: tunnel_key nofrag (skip_hw)                                   [ OK ]
 # INFO: Could not test offloaded functionality
 ok 1 selftests: net/forwarding: tc_tunnel_key.sh

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>