Move lcov-filter.py to cmake/utils

Summary:
Since coverage is a test related notion, this seems a better place.
The final goal is to include it with the secp256k1 library, and the
contrib directory is conflicting.

Test Plan:

../configure --enable-lcov
make
make cov

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5319