test: use portable EOL

The test wanted to cut huge string into 1KB strings,
for which a new line character was inserted at appropriate
places. The value is different in Windows (10, 13).
Make it portable, by making use of os.EOL semantics

Refs: https://github.com/nodejs/node/issues/25988

PR-URL: https://github.com/nodejs/node/pull/32104


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>