test: cope with uutils coreutils flag parsing for date command

test: cope with uutils coreutils flag parsing for date command

With GNU coreutils, one can pass an argument to the "set" flag like:

 date -s "-3 days"

With uutils coreutils, that gives an error:

 error: unexpected argument '-3' found
    tip: to pass '-3' as a value, use '-- -3'

That's a bug (https://github.com/uutils/coreutils/issues/9679), but both
implementations accept --set="-3 days" just fine, so use that instead.