Upgrade test-ubu1204-5 to MySQL 5.7
This will lead to an arrangement with:
-
test-ubu1204-1
-- Nightly matrix test runner, older stack (PHP 5, MySQL 5.5) -
test-ubu1604-1
-- Nightly matrix test runner, newer stack (PHP 7, MySQL 5.7) -
test-ubu1204-5
-- PR test runner, intermediate stack (PHP 5, MySQL 5.7)