testing: replace all GOOS-specific path separators in TempDir

testing: replace all GOOS-specific path separators in TempDir

For GOOS=windows the path separator characters '\' and ':' also need be
replaced.

Updates #38465

Change-Id: If7c8cf93058c87d7df6cda140e82fd76578fe699
Reviewed-on: https://go-review.googlesource.com/c/go/+/229837
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>