Unescaped URLs are misinterpreted by sprintf, giving warnings on test runs

I've working on updating the tests for Organic Groups, and ran into this issue: with a URL like http://newdrupal.dev/og/approve/652/725/7dcf51baadc7acf6296a8adbef8ced91?destination=og%2Fusers%2F652, the character sequence %2F gets interpreted by sprintf as a 2-precision float in line 312 of simpletest/test_case.php. And since there isn't a large array which substitutes in it's value, the test fails with exceptions. This is a simple patch that escapes this kind of URL by just passing %s in URLs as HTML entities.