codecept_output_dir() might not exist when the PHP-serialized coverage
report is being generated. phpunit/php-code-coverage >= 6.0.8 fix this
by creating that directory before writing the coverage report.
PHP 5.6 can only get phpunit/php-code-coverage up to version 4.0.8,
which does not have this fix. A workaround has been introduced in this
commit to allow PHP-serialized coverage reports to be stored with PHP
5.6.