Use cstdint in generated test headers

Summary: It is more in line with the style we are adopting.

Test Plan:

make clean
make check

Reviewers: #bitcoin_abc, markblundeberg, Fabien

Reviewed By: #bitcoin_abc, markblundeberg, Fabien

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3574