Tests: Replace hardcoded `/tmp/` references with `get_temp_dir()`.

This allows more tests to pass on Windows.

Props danielhuesken, DJPaul, christophherr, joemcgill, netweb, davidbaumwald, SergeyBiryukov.
Fixes #40856, #39975.
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git-svn-id: https://core.svn.wordpress.org/trunk@48233 1a063a9b-81f0-0310-95a4-ce76da25c4cd