This change adds //go:build integration to all files in

integration_tests/ so they are excluded from go test ./... by default.

Run them explicitly with go test -tags integration or make
integration-tests.
This simplifies the Makefile test target — no more shell pipeline to

filter out the integration_tests package.
While in here, I've also made it so that the test targets use

gotestsum to give much, much nicer output while running tests.

Ran unit tests.
Ran integration tests.