Don't remove window.__synthTest

In Firefox JavaScript inside an href (for example:
    `href="javascript:__synthTest()"` fires well after Syn has loaded.
    Instead of trying to predict when it is "safe" to remove this
    function, we're just going to leave it alone as it is double
    underscored and shouldn't conflict with anything. In the future we
    could create a `noConflict` function that removes this. Fixes
    lingering issue in #30