Fill spatialLayers[] for VP9 and AV1 in codec tests.

Required since https://webrtc-review.googlesource.com/c/src/+/374661. Otherwise encoding multiple spatial layers (for example, --scalability_mode=L3T1_KEY) is not working.

Also replaced accumulate() with a loop in a code that calculates total spatial layer bitrate. accumulate(first, last) works over [first, last) while we need last to be included.

Bug: webrtc:42225151
Change-Id: I266dd1b91f1f86c8ef83369c2ddf6f9a44c7f2a7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/394982
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Auto-Submit: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44840}