[enhancement]: Add itest-race target in Makefile

[gijswijs]

Background

Itest fail sometimes on CI while passing locally. This led to a discussion about making make itest-race part of the CI.
Whether we do add it to CI or we don't, having the Makefile target makes sense. The decision to add it to the CI can be made at a later stage.

Deliverables

Makefile target itest-race similar to the homonymous target in lnd.

[gijswijs]

Would we need to build the lnd binary with the -race flag? Or only run go test with the -race flag?