gh-94808: add tests covering `PySequence_[InPlace_]Repeat`

[sobolevn]

Things I've skipped:

1. NULL checks for PyLong allocation and NULL check for self. Cannot be reproduced from python
2. Different values and corner cases for sq_repeat and sq_inplace_repeat, I expect them to be covered in * and *= operators

Coverage view before:

• Issue: gh-94808

[sobolevn]

Merge conflicts are resolved, @serhiy-storchaka would you mind taking a look? :)