test: refactor mkdtemp test and added async
This test refactored the original test for mkdtempSync prefix validation and added the test also for the async function mkdtemp.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows [commit guidelines][]
Affected core subsystem(s)
test fs