mv: gnu test case `mv-n` compatibility

[matrixhead]

This GNU test checks whether mv -n works as expected, and it was failing for a number of reasons.

reason 1:

touch a b
mv -vin a b

In GNU's implementation, this would fail silently without giving any error, but uu-mv would exit with an error saying: mv: not replacing 'b'

reason 2

mv -b --update=none  a b

In GNU's implementation, this would fail with an error saying mv: cannot combine --backup with --exchange, -n, or --update=none-fail, but in uu-mv's case, it would fail silently.