net/http: Too many goroutines running after net/http test(s)

```
#!watchflakes
post <- pkg == "net/http" && test == "" && `Too many goroutines running after net/http test`
```

Seems to have started ~2 days ago https://github.com/golang/go/issues/66221#issuecomment-4229786955
This is also pretty common in presubmits.