test: fix test-buffer-slow

Fix incorrect use of string instead of RegExp in `throws` assertions.

PR-URL: https://github.com/nodejs/node/pull/9809


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>