Fix postgres processor specs

Fix bug in the gems/gitlab-active-context/spec/lib/active_context/databases/postgresql/processor_spec.rb tests:

By stubbing relation.to_sql to return expected_sql and then comparing the result against the same expected_sql, the test will always pass even if the generated SQL differs from what we expect. This defeats the purpose of the test, which should verify that the processor actually generates the correct SQL.

This can be reproduced by changing any of the expected_sql statements to something incorrect and the test still passes.

We need to update this test so that we verify the actual SQL is being built correctly.

original issue: Convert AST to postgres query (#508637 - closed)