Discourage `CHECK_IS_TEST` in *ForTesting functions.

There is already a presubmit test which blocks adding calls to test-only
functions to production code paths and adding `CHECK_IS_TEST` can make
trivial test-only accessors non-trivial, reducing readability.

Change-Id: I0a18f1bdd4da10cd690d043fd96dd69d38c70e9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6480714
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Solomon Kinard <solomonkinard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450311}