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Constructs a test private certificate with the input visibility. This
will be useful for certificate manager fakes and unit tests that
require certificates with different visibilities.

Change-Id: Ibd3e71d51d06e38c4029a6dbfc3653cc57dbef97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311048
Commit-Queue: James Vecore <vecore@google.com>
Auto-Submit: Josh Nohle <nohle@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#790872}