Revert "Revert "test: fix t7001 cp to use POSIX options""

This reverts commit 6d3e23d45cb224cbfac9d0c379cdc91d7fa63c0c.

Our cp, GNU coreutils 5.97, from the new git-for-windows sdk is
recent enough to understand -P properly.

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>