Make `go test` command replaceable in Makefile (#4934)
Make `go test` command replaceable in Makefile (#4934)

There are multiple drop-in replacements for the `go test` command, such
as [gotest](https://github.com/rakyll/gotest) or
[gotestsum](https://github.com/gotestyourself/gotestsum).

Setting the `GOTEST` environment variable allows for replacing the `go
test` command in the `test` make target to use the specified test
runner.

Usage:

```bash
GOTEST=gotest make test
```

Signed-off-by: Christian Haudum <06bf21b9be0da05e3dc60dceb5b68481e4c9f36a@gmail.com>
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki