Add fixers for refactorings due to phpunit 7/8 deprecations

[phil-davis]

See owncloud/core#36501 for description of the phpunit deprecations and associated php-cs-fixer fixes.

Add these to the regular code-style checks so that people who add new unit tests will realise to use the newer phpunit functions/methods...

In composer.json require friendsofphp/php-cs-fixer 2.16 or later, just to make sure that all the fixers will be available.

Make this version 2.0.0 of coding-standard so that we can implement it in a controlled way - the existing version 1.0.* will continue to be used by core and app repos until we purposely change the composer.json in core and each app. At that time we would also use make test-php-style-fix to automatically apply the refactorings.

[phil-davis]

@micbar do you know how the release process happens here to make 2.0.0 and have it get onto packagist or wherever so that our CI will find it?

[micbar]

not sure
@PVince81 @DeepDiver1975

[phil-davis]

@DeepDiver1975 ping
Your smiling face appears as the maintainer at https://packagist.org/packages/owncloud/coding-standard

[phil-davis]

The "magic" is just to make a release (which will also add a tag) here in GitHub. After 15 minutes or so, packagist will find it and publish it on packagist.