benchmark: use unique filenames in fs benchmarks
Use a unique file name for each benchmark. Running benchmarks
simultaneously may be a bit of an unusual use case, but there are use
cases, such as stress testing test/parallel/test-benchmark-fs.js
.
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
benchmark fs test