selftests: openvswitch: Add set() and set_masked() support.

These will be used in upcoming commits to set specific attributes for
interacting with tunnels.  Since set() will use the key parsing routine, we
also make sure to prepend it with an open paren, for the action parsing to
properly understand it.

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/20240625172245.233874-4-aconole@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>