Harden long string testing

With small string optimization csringhofer pointed out that most of our test data have small strings. And new features are typically tested on the existing test tables (e.g. alltypes that only have small strings), or they add new tests with usually small strings only. The latter is hard to prevent. Therefore the long strings might have less test coverage if we don't pay enough attention.

To make the situation better, we could

1. Add long string data to the string column of alltypes table and complextypestbl and update the tests
2. Add backend flag the makes StringValue.Smallify() a no-op, and create a test job (probably with an ASAN build) that runs the tests with that flag turned on.