Increase the code coverage

[jakubtobiasz]

Currently with the following configuration:

    <coverage>
        <include>
            <directory suffix=".php">src</directory>
        </include>
        <exclude>
            <directory>src/Api/Doctrine</directory>
            <directory>src/Api/Resource</directory>
            <directory>src/Controller</directory>
            <directory>src/DependencyInjection</directory>
            <directory>src/Fixture</directory>
            <directory>src/Form</directory>
            <directory>src/Model</directory>
            <directory>src/Test</directory>
            <directory>src/Twig</directory>
            <file>src/CommerceWeaversSyliusTpayPlugin.php</file>
        </exclude>
    </coverage>

our code coverage is about 80%. As we always aim higher in terms of quality, we should at later point take care about this metric.

Small wins 🏆

• Total code coverage is at least 90%
• This extension is installed
• CI reports if the code coverage is below 90%

[arti0090]

#106 in the time of creation solved 1st and 2nd task from TODO list. I didn't do anything regarding CI on it.
As it was some time ago, rebase will be needed and possible adjustment but overall this PR should solve quite a bit of this task