`cp -p` fails to preserve group, if user can't be preserved

[drinkcat]

Part of tests/cp/preserve-gid.

Simplified:

sudo rm c1*
touch c1
sudo chown root:wheel c1
cp -p c1 c1-gnucopy
cargo run cp -p c1 c1-uucopy
ls -l c1*

Output:

 rw-r--r-- 1 root     wheel    0 Jul 24 16:48 c1
-rw-r--r-- 1 drinkcat wheel    0 Jul 24 16:48 c1-gnucopy
-rw-r--r-- 1 drinkcat drinkcat 0 Jul 24 16:48 c1-uucopy

c1-uucopy should have wheel group.

2 notes:

• It's ok for the group to be reset to default if the user is not a member of it (I picked wheel because drinkcat is part of it). root:11234 is reset to drinkcat:drinkcat.
• This works fine with drinkcat:wheel.