Stop processing of JavaScript after phantom.exit().

This code:

    console.log("Hello World!");
    phantom.exit();
    console.log("Meh, noone should see me!");

currently produces this unexpected output:

    Hello World!
    Meh, noone should see me!

This patch fixes it to only output the first line, by loading a blank
page on phantom.exit(). Direct deletion of the web page can trigger
crashes, so this is a safe workaround.

https://github.com/ariya/phantomjs/issues/12431
Add a simple test for exit behavior.

https://github.com/ariya/phantomjs/issues/12431
https://github.com/ariya/phantomjs/issues/12439