test: avoid use of xrange for compatibility

test: avoid use of xrange for compatibility

Back in Python 2 days, there were two functions `range` and `xrange`.
While `range` returned a list, `xrange` would return a generator and
thus be the more efficient variant if a big range needs to be handled.
With Python 3, the `xrange` function was removed in favor of always
returning a generator for `range`.

Convert all uses of `xrange` to `range`. While this may be less
efficient in some places for old Python versions, their support is going
to be dropped in 2020 anyway. Also, `xrange` is only ever used in the
test suite.