test: replace common.fixturesDir with fixtures module

Require `fixturesDir` from `fixtures` module instead of `common` in
test-fs-realpath-buffer-encoding.

PR-URL: https://github.com/nodejs/node/pull/16803


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