test: convert assert.equal to assert.strictEqual

converts an instance of assert.equal in a file already mostly
updated to use assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/9925


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>