Check if an assert method is static before calling it with self

[pl-github]

In my project, I have self-defined assert methods that are not static because they need to access some instance variables. But the PreferPHPUnitSelfCallRector rule converts all method calls to static calls because the names match.

I created a patch that additionally checks if the assert method can be called statically.

[samsonasik]

Looks good, thank you @pl-github