* Fix Array#[] with ArithmeticSequence with negative steps

Previously, Array#[] when called with an ArithmeticSequence
with a negative step did not handle all cases correctly,
especially cases involving infinite ranges, inverted ranges,
and/or exclusive ends.

Fixes [Bug #18247]

* Add Array#slice tests for ArithmeticSequence with negative step to test_array

Add tests of rb_arithmetic_sequence_beg_len_step C-API function.

* Fix ext/-test-/arith_seq/beg_len_step/depend

* Rename local variables

* Fix a variable name

Co-authored-by: Kenta Murata <3959+mrkn@users.noreply.github.com>