Unset the default permalink structure after installing Multisite for automated tests.

Most of our unit tests that involve permalinks expect non-pretty permalinks;
this is the default setting for WP. However, when the test suite installs
multisite, `populate_network()` is run before WP recogizes itself as multisite,
which leads to pretty permalinks being set. This breaks a number of unit
tests in ways that are difficult to trace.

git-svn-id: https://develop.svn.wordpress.org/trunk@30267 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/install.php

@@ -56,4 +56,5 @@ if ( $multisite ) {
 
 	install_network();
 	populate_network( 1, WP_TESTS_DOMAIN, WP_TESTS_EMAIL, $title, '/', $subdomain_install );
+	$wp_rewrite->set_permalink_structure( '' );
 }