os: use t.TempDir in TestReadOnlyWriteFile

This test is checking WriteFile, not MkdirTemp, and using t.TempDir
makes the test case code a tad smaller and simpler.

Change-Id: I48837f77572f375d56f3a387efa9a4ee1d3706fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/606898


Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>