Running the functional test suite can generate several GB of data. Many systems are not provisioned with a /tmp that big. Using the build folder is better. It also makes it easier to access the files when a test fails.
Running functional tests is already creating a temporary directory inside build.
root@7c09c6004b8a:~/build# test/functional/test_runner.py --extend Starting Bitcoin ABC functional tests Temporary test directory at /root/build/test/tmp/test_runner_₿₵_🏃_20200517_163544
Should we close this task?