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