Der für uns wesentliche Unterschied sollte die Verwendung von MySQL 8 sein. Auf diese Weise können wir testen, ob wir vorwärtskompatibel sind.

Update the test matrix to run also PHP 8.x while dropping compatibility with really old package versions.

Update `composer.lock` (used when building the boxed .phar version) using PHP 7.4.

The `DumperTest` class is FUBAR in its design: It mocks the DBAL connection, making assumptions about the order and semantics of the queries the `Dumper` class will issue.

As long as we need to query the database for schema information and/or use the DBAL schema tool that does it, probably the only way to go for code like this is to do real functional/integration tests against a real database.

Since this broken test is getting in our way in #88, let's remove it.

#88)

We are looking into adding an option to do CSV exports as well. As a first step in that direction, we need to separate everything related to how we write export SQL from things related to fetching the data and user interface. Currently, that's all mixed up in the `Dumper` class.

This PR takes a first step in that direction by introducing the `OutputFormatDriverInterface`, and moving everything related to exporting SQL from the `Dumper` class to `MysqlOutputFormatDriver`.

Co-authored-by: Matthias Pigulla <mp@webfactory.de>

This PR contains a few refactorings that make the `SlimdumpCommand`, `DumpTask` and `Dumper` classes and method signatures more straightforward:

* By creating the `MysqlOutputFormatDriver` in `SlimdumpCommand`, we can pass it the necessary information from command line switches directly. No need to pass-through these flags and settings through other classes/methods.
* Separate the `OutputInterface` given to `MysqlOutputFormatDriver` (for writing out SQL) from the `OutputInterface` passed to `Dumper` (where it is used to output progress information)
* Use a `NullOutput` for the progress-related `OutputInterface` instead of passing a `$noProgress` boolean through several methods. Saves us a bunch of `if`-conditionals.
* "Extract method" refactorings for some meaningful chunks of code

The `Connection::ping()` method is deprecated in Doctrine DBAL.

According to the deprecation notice, "retry and reconnecting lost connections now happens automatically".

Thus, we can remove the `keepalive()` method.

Downgrade dependencies to versions compatible with PHP ^7.2, and add a workflow to check if the composer.lock file works for different PHP versions.

We need to declare `symfony/service-contracts` `^2.1.2` to come up with dependency versions compatible with PHP 7.2 – 8.1.

This change implements a new output mode for CSV files.

The new command option `--output-csv=...` enables CSV output mode, and must be given the path to a directory. The output mode will create `.csv` files in the given directory, named according to the dumped tables.

This output format does not support output redirection from `stdout` as the default MySQL SQL format does.

CSV files will be created only for tables that contain data. In other words, `schema` type dumps will skip the table in question. Also, dumping views/triggers makes no sense for CSV files, they will be skipped as well.

How to best write binary (BLOB) data and/or NULL values in CSV files is probably highly controversial. For now, we'll just go with the [`0x...` hex literals supported by MySQL](https://dev.mysql.com/doc/refman/8.0/en/hexadecimal-literals.html). Maybe having binary data in CSV files is not a sane idea in the first place.

Co-authored-by: Matthias Pigulla <mp@webfactory.de>