test: improve assertion in test-require-dot

Include the value that differed from the expected value in an assertion
message in test-require-dot.

PR-URL: https://github.com/nodejs/node/pull/16805


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>