Parsing equal queries with Bitwuzla results in non-equal formulas

Parsing 2 equal Strings to formulas results in 2 non-equal formulas. It seems like variables are not reused, but created anew with each query.
We should ask the Bitwuzla devs if this can be improved.

Since this and #329 hinders a lot of tests, i've temporarly disabled the Bitwuzla parsing tests.