test improvements (#5741)

## Summary
Some test improvements:

- disable the race detector on macos, which currently results in a ton
of linking errors, and I think doing race detection on linux is
sufficient to find stuff
- add build tags to the `integration` folder so it's not included by
default when running `go test ./...`
- remove some unused stuff in the Makefile
- change the file `testenv` searches for from `.git` to `go.mod` which
allows test caching to work more reliably
- remove the test for orphaned connections in postgres. It doesn't
actually make sense because we have listener connections in the
background. I think it was racy.
- fix yarn caching


## Checklist

- [ ] reference any related issues
- [x] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

integration/authentication_test.go

@@ -1,3 +1,5 @@
+//go:build integration
+
 package main
 
 import (