Since phpunit v9 a trait was introduced to provide $this->prophesize() function, that
is used extensively throughout the TYPO3 core. With usage of phpunit v9, a
deprecation warning is raised for each call to $this->prophesize().
The package providing this trait is phpspec/prophecy-phpunit,
which requires PHP 7.3 minimum. TYPO3 v10 still supports PHP 7.2, so the
package can not be used.
To avoid the deprecation warning, the ProphecyTrait has been included into the
TYPO3 Testing Framework.
Once TYPO3 drops the support for PHP 7.2, the imported trait class can be removed
and the original package required.