docs: Add a note about thin testers to the infra glossary

Clarifies the term "thin tester" which is kind of a sub category of
"child tester".

Bug: None
Change-Id: Ia83c8d90c6c1ea3d96fe8ea6b8969c0abd2c18dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6203042
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411787}