Simplify implementation

[chapterjason]

List of the most important changes (From the commits but without Fix CS stuff commits.):

• refactor!: Rename addCompilerPasses to addCompilerPass
  BREAKING CHANGE: Renamed addCompilerPasses to addCompilerPass, this also changed the signature to single compiler passes only.
• refactor!: Rename setRoutingFile to addRoutingFile
  BREAKING CHANGE: Renamed setRoutingFile to addRoutingFile to allow multiple routing files
• refactor!: Rename addConfigFile to addConfig
  BREAKING CHANGE: Renamed addConfigFile to addConfig cause it also takes a closure to allow configuration with the Symfony\Component\DependencyInjection\ContainerBuilder
• refactor!: Rename AppKernel to TestKernel
  BREAKING CHANGE: Renamed AppKernel to TestKernel to make it more obvious what the kernel is used for.
• refactor: Switch to configure kernel class via method instead override static property
• refactor: Use MicroKernelTrait instead of custom implementation
• refactor: Use yield instead of temporary array
• fix: Add realpath for cache and log dir
  This resolves symlinks that might be used in systems like MacOS
• cut: Remove setRootDir method
  The rootDir is already deprecated lower than symfony 4.4, no need to support.
• ci: Update to latest ubuntu
• chore: Add types on several places
• chore: Update php-cs-fixer

Propery and method names

I also thought about changing the properties something like:

    private array $testBundles = [];
    private array $testConfigs = [];
    private array $testRoutingFiles = [];
    private array $testCompilerPasses = [];
    private string $testCachePrefix;
    private ?string $testProjectDirectory = null;

This will ensure they will, should, never conflict, and it helps while maintaining to differ between Symfony properties and own properties.

Also for the related methods I would prefix them like addTestRoutingFile, also this in the public api makes it more obvious for the developers what they are working with.

WDYT?

Readme

Using this bundle test together with Matthias Nobacks's
SymfonyDependencyInjectionTest
will give you a good base for testing a Symfony bundle.

Theres still that mention in the readme, I don't think that this will still be compatible.
I think we could also refactor a lot there to traits like ContainerAssertionsTrait

Also for the ConfigTest package.

Related to #55

[Nyholm]

Great work.

I like this.
Im happy to merge this now and then update readme and properties/methods in another PR.

[Nyholm]

Thank you