Add some tests for asynchronous CertVerifiers.

Fill some of these in in preparation for the implementation changing.
Test that both success and failure may be reported asynchronously, and
also test that the CertVerifier::Request machinery works.

Bug: 347402
Change-Id: I8f68c1f5be9409ee8643ac8bc951ebf7e1fd12c3
Reviewed-on: https://chromium-review.googlesource.com/c/1372338
Reviewed-by: Steven Valdez <svaldez@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617190}
3 files changed