Test every dialect in query builder unit test

The querybuilder tests in unit/query/builder.js mostly only tested the postgres syntax, because I'm working on non-quoting option I needed some more tests. So I changed the querybuilder test to be more modular and to run all tests for all dialects.

Some problems for sqlite3, some non-supported oracle syntax and an offset() without limit() problem for all but oracle came up. Problematic cases are now only marked with a TODO in favor of the passing test for the pull request. I'll try to fix the bugs and send a separate pull request.

If it's not obvious with the code I can add some explanation how it works/how to add tests.