infra: Add basic builder specs for the two UTR-tester builders.

These specs won't really change much (if at all). And other than
maybe one more mac builder, this recipe won't run on very many builders.

So this change is sorta silly. Really it's only intent is to make
led testing an entire builder's suites via these UTR-tester recipes
much easier. Today, one can craft a led request to have one of the
UTR tester bots run a random bot's suite. But if that bot's gclient
config doesn't match what the utr-tester recipe hardcodes, then the
results might not be reliable.

So by specifying the builder-spec in input props, the led request
can go in and also tweak these specific input props to change the
builder spec as needed to match the bot. We assume this is easier
than also modifying the recipe via `led edit-recipe-bundle` in the
request.

A subsequent recipe change will be needed to make the recipe read
from these input props.

Bug: 349428212
Change-Id: Iacac01f81fbaf84ad6a732f11114c37c2788eff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5662245
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Cr-Commit-Position: refs/heads/main@{#1320549}