test: put expected assert value in correct place

The order of arguments in test-fs-readdir-ucs2 was the
opposite of what is recommended in the documentation,
which is that the first value should be the value being tested,
and the second value is the expected value.
This fixes the test to make it conform to the documentation.

PR-URL: https://github.com/nodejs/node/pull/23505


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>