xfstests: fix make install race condition

* sometimes make install was failing with:
  cp: cannot stat 'group.list': No such file or directory
  and bunch of non-fatal messages:
  mv: failed to preserve ownership for 'group.list': Invalid argument

* this was when tools/mkgroupfile did
  mv -f "$new_groups" "$groupfile"
  overwritting the group.list file while install-sh was already
  copying it to output

* in the end easily reproducible by
  1) removing tests/*/group.list before each make install
  2) adding some sleep in mkgroupfile before the mv call

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

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