test: remove literal error messages

Assertions will now print the values that caused the assertions
to fail.

PR-URL: https://github.com/nodejs/node/pull/15928


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>