Call server.Shutdown() as go routine (#33288)

server.Shutdown() blocks trying to write to s.shutdownFinished in some cases when using a test Grafana instance for a series of integration tests. 

See https://github.com/grafana/grafana/blob/master/pkg/server/server.go#L222-L224

pkg/tests/testinfra/testinfra.go

@@ -58,7 +58,7 @@ func StartGrafana(t *testing.T, grafDir, cfgPath string, sqlStore *sqlstore.SQLS
 		}
 	}()
 	t.Cleanup(func() {
-		server.Shutdown("test cleanup")
+		go server.Shutdown("test cleanup")
 	})
 
 	// Wait for Grafana to be ready