[CI] Add an option to build and run unit tests with debug enabled

Summary:
Calling the script with ENABLE_DEBUG=1 will build and run the unit
tests with debug enabled.
Functional tests are known failing under debug, they will be skipped.

Test Plan:

ENABLE_DEBUG=1 ./contrib/teamcity/build.sh

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2776