#6622 Logger refactoring: introduced a DI config for the test environment
#6622 Logger refactoring: moved the container creation into a new ContainerFactory

This class can be used in tests to create a specific environment (e.g. the prod environment, CLI environment, etc...)
#6622 Logger refactoring: cleared up formatting that was left in FileHandler

This is so the FileHandler can be replaced by Monolog's StreamHandler
#6622 Logger refactoring: Piwik FileHandler now extends Monolog's StreamHandler

We can't remove FileHandler completely because it generates a custom exception message in case of error.
#6622 Logger refactoring: Simplified StdErrHandler now that logging is disabled in tests
Added test for ClassNameProcessor
#6622 Logger refactoring: RequestIdProcessor doesn't need static variables now that we have a container
#6622 Logger refactoring: added tests
#6622 Logger refactoring: Turned the Exception (text) formatter into a processor + added tests

Monolog handlers support 1 formatter. Using processors instead of nested formatters leads to much simpler code.
#6622 Logger refactoring: StdErrHandler: had to revert a previous change

In the tests, logging is disabled. But not when testing the logger itself…