proposal: add a goroutine leak detector to go test

[jeanbza]

Inspired by grpc-go's leak detector (example usage here) by @menghanl and co.

Leaking goroutines is a problem that's hard to spot. We can help users detect these goroutines by adding a -leak flag to go test that detects leaking goroutines. A rough outline of the lifecycle of a test in the presence of this flag is as follows:

1. Count non-test, non-runtime goroutines
2. Run the test
3. Wait some period of time for things to shut down (possibly configurable with a default of 100ms, for instance)
4. Count non-test, non-runtime goroutines
5. If before != after, err

[jeanbza]

Dupe #5308 / #6705

[jeanbza]

Other impls, for posterity:

• https://golang.org/src/net/http/main_test.go#L60
• https://godoc.org/github.com/anacrolix/missinggo/leaktest