Split IntegrationSpec (#1535)

This change makes `IntegrationSpec` an abstract class trait that defines
common utilities used in most integration tests, and splits the actual tests
in two separate files that extend that class. Channel tests have themselves
been split between standard and anchor commitments to gain extra
parallelism.

The tests themselves aren't changed at all, except for the TCP ports they
use, and to remove nodes that have become useless in each of the specs. `expectMsg`
have been increased to 60 seconds to account for the increased parallelization.

Some tests have been refactored, cleaned up and block expiries have been reduced.