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.
This is a decision annouced by @MarkBaker in Gitter to be able to
use `DateTimeInterface`.
Also remove deprecated composer option, and introduce caching for
faster CI tests
Adrien Crivelli