Add support for xdebug in WebTestBase

[klonos]

This is to crossport https://www.drupal.org/node/889338 which was backported from D8 to D7 core back in 2015:

Since simpletest is meant for testing/development, it would be helpful if it fully supported debugging.

As things are now, if you want to debug what happens inside the drupalPost/drupalGet requests in Simpletest [debug in the sense of "step through the code in a debugger", such as xdebug or Zend], you have to do something like what is described on the bottom of this page:
http://drupal.org/node/30011

Namely, you either need to set up a separate Apache virtual server and use that, or you need to hack drupalPost/drupalGet to add something like this near the top to add the debugging session information to the URL before it is passed through curl:

if (!isset($options['query'])) {
  $options['query'] = array();
}
$options['query'] += array('XDEBUG_SESSION_START' => 'ECLIPSE_DPGP');

It would be nice if SimpleTest could do this automatically. ...

PS: noting that there is this D8 -> D7 backport follow-up in d.o: Make the Simpletest XDebug integration work for CLI requests. If we take too long to review/test this, perhaps keep an eye on that other issue and include it here.

[quicksketch]

I used xdebug all the time with SimpleTests without any special work. But I think ddev might be doing something to make that easier. Checking out the php.ini settings it adds, it sets xdebug.start_with_request=yes, which means xdebug runs on every request. If set to xdebug.start_with_request=trigger, then XDEBUG_SESSION_START is used to start xdebug on a conditional basis. But I think this is all only feasible with ddev because it makes it so easy to turn on/off xdebug:

ddev xdebug on

...run my debugger requests...

ddev xdebug off

It looks like xdebug.start_with_request=trigger is the default when xdebug is in debug mode.