[ZT] Make KeyRotationManagerTests Run Again

Put back the test target, but changed the "installer" target name which appeared to have been conflicting with the top level "installer" target, which led to CI builders breakage (see crbug.com/1262172). The issue has been mitigated in another CL (crrev.com/c/3237749), but the tradeoff was that the new unit tests wouldn't run anymore (the current CL fixes that).

Change-Id: I86a19a5915967c7addff880ef10da338e34d773d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3237710
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#934502}