Tests: Improve setup of fake cursor within DialectTest test cases
Tests: Improve setup of fake cursor within DialectTest test cases

The test cases use a shared instance of a fake cursor, which is needed
to satisfy CrateDB version inquiry of the driver early on, so it needs
to be established _before_ invoking `sa.create_engine()` / 
`engine.connect()`.

On the other hand, the _individual_ configuration of the fake cursor
should happen per test case, to be able to define different values
for `cursor.return_value` _and_ `cursor.description`.

Beforehand, the corresponding test cases only succeeded because all used
the same structure of a database response, i.e. all queried only a
_single_ data column. Now, response structures are defined individually
by setting `cursor.description` within the body of each test case.
crate/crate-python,crate/crate-python