ocsp: rewrite tests that rely on SHA1-RSA

This test suite should really be completely re-written, but for now
we can just substitute the tests which relied on certificates that
were signed with SHA1-RSA.

Fixes #49443

Change-Id: Ibe4ae3b3062956a56e6e3579144312747df3ef9a
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/362334
Trust: Roland Shoemaker <roland@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Katie Hockman <katie@golang.org>