test: network_topology_strategy_test: fix overflow in d2t()
test: network_topology_strategy_test: fix overflow in d2t()

d2t() scales a fraction in the range [0, 1] to the range of
a biased token (same as unsigned long). But x86 doesn't support
conversion to unsigned, only signed, so this is a truncating
conversion. Clang's ubsan correctly warns about it.

Fix by reducing the range before converting, and expanding it
afterwards.

Closes #7376

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla