test: improve assert message

Use the default assert.strictEqual() message so that unequal values are
shown in the event of an AssertionError.

PR-URL: https://github.com/nodejs/node/pull/19629


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>