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

Closes #680