Clean up verifier testcases.

A Certificate is a pair of an RSAPublicKey and a particular hash. So v1
and v3 differ in the hash algorithm (SHA-1 vs SHA-256), similarly for
v2 and v4.

In verifier testcases, we used to load v1/v2 keys with an explicit
argument of "sha256" to test the v3/v4 keys. This CL switches to loading
v3/v4 keys directly and lets load_keys() to handle that, which is the
actual flow we use in practice.

Also remove the "fallback to v1 key" in the testcases, which is not the
actual behavior.

Bug: 30415901
Test: Run the verifier_test component test on device.

Change-Id: I3a2baa64826f1b6c4c367a560090df384c4521bb