test: remove assert message and add block scope

The default message will be printed if the assertion fires. Use block
scope for related variables and tests.

PR-URL: #19054
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: wuweiweiwu