dev/sg: make 'sg test' operate based on current dir

Created by: bobheadxi

similar to cd my-dir && go test

We currently only run in root: https://sourcegraph.com/github.com/sourcegraph/sourcegraph@a4b19528469aa1453c4134fa0b96112f2414707c/-/blob/dev/sg/internal/run/run.go?L601%3A1-607%3A93=