Fix QgsSymbolLayerV2 test

The code in `functionFromSldElement` looks for a element with the name `Filter` If there is none found, the method returns.
In the testdata, there is no `Filter` defined, just a `Literal` which because of the aforementioned condition never gets parsed.

This PR more or less reverts parts of 3e98d7d8 which is not necessarily a good idea, but I fail to see the proper way of handling this.