TextOperationWithFilter with width > 1 is untested in End-to-End Tests

[niklas88]

In #240 I added an End-to-End test checking for correct TEXT(?t) and also one that uses TextOperationWithFilter. However that only uses a filter with 1 column. I tried getting the optimizer to use TextOperationWithFilter with more than 1 column but wasn't successful.

[hannahbast]

This code has been refactored significantly. In particular, the idiosyncratic TEXT function no longer exists. Such queries can now be asked with SPARQL-conform syntax.