ansible-test/split_targets: improve the start/end computation

[goneri]

Simplify the logic and correctly compute the end of the first job
in case the number of targets is lower than ansible_test_split_in.

[goneri]

recheck

[goneri]

recheck

[oukooveu]

are you sure this will work as expected? for example for ansible_test_split_in = 3 and number_entries = 10 this produces ranges: 0-4, 4-8 and 8-12.

i can suggest following change:

_end_at: "{{ _start_at|int + _iter_by|int - 1 if ansible_test_do_number|int == _iter_by else number_entries - 1 }}"

and this produces 0-3, 4-7 and 8-9 for the same input.