Prior to PG 10.4, the costing for hashaggs was different (see
PG commit `1007b0`). We fix the tests not to change between the
old and new versions by disabling hashagg. We were not testing
for that anyway.
This PR adds test infrastructure for running tests with shared tables.
This allows having hypertables with specific configurations usable for
all tests. Since these tests also don't require creating a new database
for each test case some of the overhead of the normal tests is removed.
While this will lead to much faster query tests some tests will still
require their own database to test things, but most queres could be moved
to this infrastructure to improve test coverage and speed them up.
