unix: add SockaddrUnix tests on darwin and *bsd

Change-Id: I65b4c1073208079ea5ac28d21c829ecf28e28d84
Reviewed-on: https://go-review.googlesource.com/c/sys/+/229878
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_internal_linux_test.go

@@ -460,7 +460,6 @@ func TestSockaddrUnix_sockaddr(t *testing.T) {
 			}
 		})
 	}
-
 }
 
 // These helpers explicitly copy the contents of in into out to produce