Bootstrap Drupal in bootstrap-phpunit, not setUp()

PHP's error handler logic is very dumb.  We need to restore_error_handler()
after bootstrapping Drupal, but we don't want to call restore more than
once, as that will get rid of PHPUnit's error handler too.

drupal_bootstrap has a static var tracking what it has done, so it only
takes over the error handler the first time it is called.

This patch moves Drupal bootstrapping and restore_error_handler into
the phpunit bootstrap file so it's only called once.

Change-Id: I89c8d52fd22a61dfc8e9cc586ee43027007bec55
Merge "Bootstrap Drupal in bootstrap-phpunit, not setUp()"