Hopefully the last refactoring of TestRespectGroupSticky:
* Properly tested (+simplified) FreeBSD fix
* Tested on Darwin (10.12.4)
* Rename to TestRespectSetgidDir (I believe this is the accepted
  terminology)

Fixes golang/go#19596.

Change-Id: I8d689ac3e245846cb3f1338ea13e35be512ccb9c
Reviewed-on: https://go-review.googlesource.com/41430Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>