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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines