[v6.x backport] test: change deprecated method to recommended

[Trott]

In non-buffer tests, change usage of the Buffer constructor to one of
the recommended alternatives.

PR-URL: #13649
Reviewed-By: Refael Ackermann refack@gmail.com
Reviewed-By: Colin Ihrig cjihrig@gmail.com
Reviewed-By: Luigi Pinca luigipinca@gmail.com
Reviewed-By: James M Snell jasnell@gmail.com
Reviewed-By: Tobias Nießen tniessen@tnie.de
Reviewed-By: Brian White mscdex@mscdex.net

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)