Add string addition perf test
Add string addition perf test

svaarala/duktape,markand/duktape,svaarala/duktape,markand/duktape,svaarala/duktape,markand/duktape,markand/duktape,svaarala/duktape,markand/duktape,svaarala/duktape,markand/duktape,markand/duktape,markand/duktape,svaarala/duktape,markand/duktape,svaarala/duktape,svaarala/duktape,markand/duktape,svaarala/duktape