[CodeQuality] Rule to enforce test method code style

[Xammie]

This adds two rules to convert the name of test methods to either CamelCase or snake_case.

// When using
use Rector\PHPUnit\CodeQuality\Rector\ClassMethod\PreferTestsWithSnakeCaseRector
$rectorConfig->rule(PreferTestsWithSnakeCaseRector::class);

// From
public function testItWillCreateNewProduct() {}
// To
public function test_it_will_create_new_product() {}