TestWriteFileModeAppliesUmask and TestVectoredHandlerDontCrashOnLibrary
could both leak /tmp/go-build-* directories which isn't very friendly.

Change-Id: Ibee9c33d49ad48958fae4df73853b82d92314bf0
GitHub-Last-Rev: 814e2fa4bb4e4fe9c00b6d465313ce35c7ab4e32
GitHub-Pull-Request: golang/go#34253
Reviewed-on: https://go-review.googlesource.com/c/go/+/194880
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>