constexpr-ify unit tests more

removing some of the more common patterns from
  git grep "^const .* k" | grep unittest.cc  | awk '{print $2}' | sort | uniq -c | sort -b -n

typically done using
  git grep "^const something k" | grep unittest.cc | awk -F ':' '{print $1}' | sort | uniq | xargs sed -i 's/^const something k/constexpr something k/g'

Bug: webrtc:429563452
Change-Id: I52fcbebd0ad8a6e98879107d7923be51da06f802
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/400400
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@meta.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45150}