test: fix flaky parallel/test-fs-write-file-typedarrays
Using the same filename for different async tests could lead to race conditions.
Example failure: https://travis-ci.com/nodejs/node/jobs/143351655
Refs: https://github.com/nodejs/node/pull/22150
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines