Core: change url() helper to output `?foo` instead of `?foo=true`

Since the URL params parsing mechanism in QUnit treats the missing value as if the param has value of `true`, we can do the same thing in reverse. If the parameter to serialize a URL with has the value of `true`, it's safe to skip the value part of the query string.

Thus, toggling the checkboxes in the toolbar will result in:

    test.html?noglobals&notrycatch

instead of:

    test.html?noglobals=true&notrycatch=true

Goes well with #678
we can reduce complexity breaking this on more statements like:

````js
querystring += encodeURIComponent( key );
if ( params[ key ] !== true ) {
	querystring += "=" + encodeURIComponent( params[ key ] );
}
querystring += "&";
```