main (test): integrate test corpus runner

This allows us to test a large corpus of external packages against the compiler.

loader/list.go

@@ -28,5 +28,8 @@ func List(config *compileopts.Config, extraArgs, pkgs []string) (*exec.Cmd, erro
 	}
 	cmd := exec.Command(filepath.Join(goenv.Get("GOROOT"), "bin", "go"), args...)
 	cmd.Env = append(os.Environ(), "GOROOT="+goroot, "GOOS="+config.GOOS(), "GOARCH="+config.GOARCH(), "CGO_ENABLED="+cgoEnabled)
+	if config.Options.Directory != "" {
+		cmd.Dir = config.Options.Directory
+	}
 	return cmd, nil
 }