[WinDT] Create windows key persistence delegate test

This CL creates the windows key persistence delegate unittest and
modifies the windows key persistence delegate to contain new static
methods for testing and setting test instances. I also modifies the
code logic to create an EC signing key when creating a TPM key fails.
This unittest is needed to add more test coverage to the device trust
connector code.

Screencast of windows key rotation: https://screencast.googleplex.com/cast/NTI1MzIzNjQ0NjQ2MTk1MnxkM2Q3ZThjNC0zZQ

Bug: b/240165293
Change-Id: I413dc64a80c9a89231491809f4d10a86011443d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3849260


Commit-Queue: Hamda Mare <hmare@google.com>
Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044774}