LibJS: Redirect stderr to /dev/null when running js in run-tests

This "mutes" output from dbg() calls - which is not an issue inside
serenity itself but if the script is run on the host machine and stdout
and stderr are displayed in the same terminal window.

Libraries/LibJS/Tests/run-tests

@@ -15,7 +15,7 @@ count=0
 
 GLOBIGNORE=test-common.js
 for f in *.js; do
-    result=`$js_program -t $f`
+    result=`$js_program -t $f 2>/dev/null`
     if [ "$result" = "PASS" ]; then
         let pass_count++
         echo -ne "( \033[32;1mPass\033[0m ) "