Refactor how the test runner uses configuration from CLI options and XML file

[sebastianbergmann]

The PHPUnit\TextUI\XmlConfiguration\Configuration object was introduced in PHPUnit 9.0. It holds the configuration loaded from an XML configuration file.

The PHPUnit\TextUI\CliArguments\Configuration object was introduced in PHPUnit 9.2. It holds the configuration passed using commandline arguments and options.

The test runner uses PHPUnit\TextUI\XmlConfiguration\Loader to load an XML configuration file and create an PHPUnit\TextUI\XmlConfiguration\Configuration object.

The test runner uses PHPUnit\TextUI\CliArguments\Builder to process commandline arguments and options to create an PHPUnit\TextUI\CliArguments\Configuration object. The test runner then uses PHPUnit\TextUI\CliArguments\Mapper::mapToLegacyArray() to map the PHPUnit\TextUI\CliArguments\Configuration object to an $arguments array that is required by the legacy code in PHPUnit\TextUI\TestRunner.

PHPUnit\TextUI\TestRunner is full of code that is hard to understand and maintain for "mixing" the configuration that comes from the XML configuration file in the form of an PHPUnit\TextUI\XmlConfiguration\Configuration object and from the commandline in the form of an $arguments array that is created by PHPUnit\TextUI\CliArguments\Mapper::mapToLegacyArray().

Replacing this overly complicated and error-prone code should be the next step in refactoring the code responsible for configuring the test runner.

[sebastianbergmann]

I started to work on this in the issue-4292/refactor-test-runner-configuration branch.