test: replace fixturesDir with usage of fixtures module

Repalce common.fixturesDir with usage of the fixtures module in
test-tls-ecdh-disable.

Add a comment explaining the test.

PR-URL: https://github.com/nodejs/node/pull/16810


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>