test,benchmark: use new Buffer API where appropriate

For tests / benchmarks that are creating Buffer instances for
any reason other than to test Buffer constructor, use the new
Buffer.alloc/Buffer.from API instead of the deprecated API.

PR-URL: https://github.com/nodejs/node/pull/18980


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>