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: nodejs#19054
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Daijiro Wachi <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>

Author: wuweiweiwu