Use runtest.cmd generate layout to unblock perf tests

Author: jashook

tests/runtest.cmd

@@ -144,6 +144,7 @@ REM runtest.py will handle setup and calls runtest.proj.
 if defined __AgainstPackages goto SetupMSBuildAndCallRuntestProj
 if "%__CoreFXTests%"=="true" goto SetupMSBuildAndCallRuntestProj
 if "%__PerfTests%"=="true" goto SetupMSBuildAndCallRuntestProj
+if defined __GenerateLayoutOnly goto SetupMSBuildAndCallRuntestProj
 
 REM We are not running in the official build scenario, call runtest.py