cmd/pack: provide executable name in TestHello

otherwise go build command adds .exe suffix

Fixes issue 7362

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/66250043
syscall: terminate error string in exec package on Plan 9
Try to prevent messages like this:
        './pack' file does not exist����������������������������������������������������������������������������������������������������
TBR=adonovan

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/66270043
net/http: remove tmp file created in TestMultipartReaderOrder

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://codereview.appspot.com/66470043
os: fix Rename on Plan 9

Rename should fail when the directory doesn't match.
It will fix the newly introduced test from cmd/pack
on Plan 9.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://codereview.appspot.com/65270044
Merge branch 'master' into gitfix