[focusgroup] Add PrintTo helper for logging FocusgroupFlags

Adds helper to make FocusgroupFlags human-readable for tests and debug
logging.

This change also updates the FocusgroupFlags parsing unittests to
prefer EXPECT_* over ASSERT_* for situations where it would be
possible for the test to continue.

Bug: 40210717
Change-Id: I07fa1d2d123f123cc2db1fafd12e9b1d8e640a58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6961756
Reviewed-by: Mason Freed <masonf@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Jacques Newman <janewman@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1519670}