Add ProbabilisticRevealTokenTestIssuer

Rename MockIssuer to ProbabilisticRevealTokenTestIssuer and
move it to its own source files. Create a testonly build target
for it.

Add `Plaintexts()` method that gives plaintext ECPoints
corresponding to `Tokens()`

This change enables using PRT test issuer from other tests.

Bug: 409597501
Change-Id: I7dcd6376f4423fee713e22926f35e6aa06c297cf
Fixed: 409769485
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6447096
Reviewed-by: Andrew Williams <awillia@chromium.org>
Reviewed-by: Ryan Kalla <ryankalla@google.com>
Commit-Queue: Aykut Bulut <aykutb@google.com>
Cr-Commit-Position: refs/heads/main@{#1446552}