tests: Change ADD_OF_PORTS from macro to shell function.

tests: Change ADD_OF_PORTS from macro to shell function.

This reduces the size of the generated testsuite and makes it possible
to pass arguments that vary at runtime instead of at the time of
translation from .at to shell script.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>