Ensure `assertM` doesn't run the passed action for non-debug builds.

Currently assertM act will always execute act even in non-debug builds.

This seems undesireable. I'm sure @bgamari will be nerd sniped into fixing this soon.

added needs triage label

mentioned in commit 897158f2

mentioned in merge request !9688 (closed)

Fixed in !9688 (closed).

added Pnormal Tbug runtime perf labels and removed needs triage label

assigned to @bgamari

mentioned in commit 4e17ebb1

mentioned in commit 14416c37

closed with commit dbbab95d

mentioned in commit dbbab95d