Simplify the test logic by using t.TempDir, t.Chdir, and Chdir to
startPath parent.

Change-Id: Ibe71a8c26b8e54c22eb93510037605b69c67bc7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/606896


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Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>