test: /postgres engine spec test coverage (#12490)
test: /postgres engine spec test coverage (#12490)

* add tests for PostgresEngineSpec.estimate_statement_cost

* add tests for PostgresEngineSpec.query_cost_formatter

* fix docstring

* fix dependency

* fix raise SyntaxError
zhouyao1994/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,apache/incubator-superset,zhouyao1994/incubator-superset,airbnb/caravel,mistercrunch/panoramix,zhouyao1994/incubator-superset,zhouyao1994/incubator-superset,mistercrunch/panoramix,airbnb/caravel,mistercrunch/panoramix,airbnb/caravel,apache/incubator-superset,mistercrunch/panoramix,apache/incubator-superset,apache/incubator-superset,airbnb/caravel