> Unfortunately I could not find any tests for the code in
> question so the only way to test it is by:

I uploaded tests recently. You can find them here:
https://gerrit.osmocom.org/#/c/933/4/tests/coding/coding_test.c

> I think the puncturing code should be part of the library (otherwise
> every user will have to reimplement the same thing again and again).

I am agree with you. We recently discussed this question with Tom Tsou.
He said, that there was some performance related reasons to perform
puncturing inside both gsm0503_pdtch_encode() / gsm0503_pdtch_decode().