selftests: openvswitch: Refactor actions parsing.

Until recently, the ovs-dpctl utility was used with a limited actions set
and didn't need to have support for multiple similar actions.  However,
when adding support for tunnels, it will be important to support multiple
set() actions in a single flow.  When printing these actions, the existing
code will be unable to print all of the sets - it will only print the
first.

Refactor this code to be easier to read and support multiple actions of the
same type in an action list.

Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20240625172245.233874-3-aconole@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>