All code for unit tests is now under the `PhpSpreadsheet\Tests` namespace
which is autoloaded via composer mechanism. So there is no need for
`require()` anymore.
Also, tests were moved in `tests/` folder and phpunit should be executed from
the project root folder. This is to conform to the de facto standard, notably
in use in phpunit itself.
Added .gitattributes file to identify files/folders that should be excluded from a composer distribution unless invoked as
composer install --prefer-source
Jean-Baptiste Quenot