[fuchsia] Explicitly set severity in logging tests

The current behavior relies on the zero values being a valid member of
the enum, which relies on a deprecated member. Explicitly setting the
severity will allow the deprecated member to be removed.

Change-Id: Ia00d6463446a8cfeb9055d7284d4d9f2292eaf87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3606917
Auto-Submit: Tamir Duberstein <tamird@google.com>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#996389}