Support dist file name phpunit.dist.xml

[mvorisek]

To be more editor friendly (IDEs highlight based on file extension) and join PHP CS Fixer which done similar change for the next major version.

[sebastianbergmann]

TBH, I would rather remove this mechanism entirely than to change it. It's a BC break either way.

[mvorisek]

Supporting one local override is helpful as the local override is preferred but can be easily ignored by VCS by default (useful for developing PRs that need a different local configuration). It is always then up too the user if he chooses to use the dist version name or not, but having this feature removed have downside.