test: eliminate port collision

In test test-cluster-net-listen-ipv6only-rr, the cluster member that
listens to `any` port actually has the potential to `grab` any port
from the environment which when passed onto the master causes
collision when it tries to listen on.

Moving the test to sequential alone is not sufficient as the cluster
member can in theory catch on to the admin ports on the host.

Assigning static port alone is also not sufficient, as it can interfere
with other running tests in the parallel category which would be mostly
running with `port: any` fashion.

So move to sequential, and use a static port.

Fixes: https://github.com/nodejs/node/issues/25813
PR-URL: https://github.com/nodejs/node/pull/26298


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>