For unit tests that call `wpmu_create_blog()`, Blog factory, or installation code that attempts to clear transients: suppress database errors on `setUp` and restore on `tearDown`.

There are a few places in core that were preventing this from working by explicity setting `$wpdb->suppress_errors` to `false`. Instead, they should inherit the value that existed before errors were suppressed.

This allows Multisite unit tests to run without explosive database errors, and allows `$wpdb->suppress_errors` to be overridden all the way down the chain.

Fixes #26102.


git-svn-id: https://develop.svn.wordpress.org/trunk@26252 602fd350-edb4-49c9-b593-d223f7449a82