test: Capitalize MUST and SHOULD directives

These directives in the API documentation have specific meanings.
Capitalizing makes this more apperant.

BUG=chromium:1063474
TEST=./generate.sh

Change-Id: Ie7577565a1600cf66fb2d52ec5f183ea4378a09b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2121573
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Alex Zamorzaev <zamorzaev@chromium.org>
Reviewed-by: Jacob Kopczynski <jkop@chromium.org>
8 files changed