make.bat: incease runtime test timeout to 300s (fixes build)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11064044

src/run.bat

@@ -43,7 +43,7 @@ if errorlevel 1 goto fail
 echo.
 
 echo # runtime -cpu=1,2,4
-go test runtime -short -timeout=240s -cpu=1,2,4
+go test runtime -short -timeout=300s -cpu=1,2,4
 if errorlevel 1 goto fail
 echo.