Allow setting an env variable to skip tests in build.sh

Summary:
This makes it easy to reuse the build.sh script for IBD without re-running the tests, saving us 20-30 minutes.
Complete test coverage would continue to be covered in other parts of the build pipeline.

Test Plan:
./build.sh # builds and runs tests
DISABLE_TESTS=1 ./build.sh # builds but skips tests

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D2752