[symfony] Cover variable and new 2nd arg in SingleArgEventDispatchRule

[TomasVotruba]

Add fixtures for dispatch() with 2 args:

• 2nd arg is a variable holding a constant string, e.g. $eventName = 'some_event' - reported
• 2nd arg is an untyped method argument - skipped
• 2nd arg is a new expression - skipped

The existing type-based check already handles all of them, these fixtures lock the behavior in.