KcerFactory: Improve shutdown for tests

In tests it is possible that KcerFactory::Shutdown() is called
before the factory finishes initializing. Stop the initialization
in such cases to prevent failure on raw_ptr checks.

Also add a couple missing calls to Shutdown() in test classes.

Bug: b/264387231
Test: *Kcer* automated tests
Change-Id: I1a90c855085112992c500e569399ccd1f2eca6ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5347716
Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Michael Ershov <miersh@google.com>
Cr-Commit-Position: refs/heads/main@{#1270160}