tests: Run custom tests on two cores
tests: Run custom tests on two cores

This is consistent with the way we run the `SEASTAR` unit tests.

To avoid duplication, we define a function for the case of a CUSTOM test
which is structured like a Seastar application.

scylladb/seastar,scylladb/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar