Overwrite permission in coreoverlay generation

fix runtest.sh to overwrite corerun's permission in coreoverlay generation

Author: hseok-oh

tests/runtest.sh

@@ -354,7 +354,7 @@ function create_core_overlay {
     mkdir "$coreOverlayDir"
 
     cp -f -v "$coreFxBinDir/"* "$coreOverlayDir/" 2>/dev/null
-    cp -f -v "$coreClrBinDir/"* "$coreOverlayDir/" 2>/dev/null
+    cp -f -p -v "$coreClrBinDir/"* "$coreOverlayDir/" 2>/dev/null
     if [ -d "$mscorlibDir/bin" ]; then
         cp -f -v "$mscorlibDir/bin/"* "$coreOverlayDir/" 2>/dev/null
     fi