test: Fix FATAL: sorry, too many clients already

By moving two Pytest fixtures responsible for database connectivity
from the "function" scope to the "session" scope, the number of
redundant invocations to `sqlalchemy.create_engine()` can be
significantly reduced. Apparently, this fixes to connection pool
overflow.

target_postgres/tests/test_target_postgres.py

@@ -47,7 +47,7 @@ def postgres_config_ssh_tunnel_fixture():
     return postgres_config_ssh_tunnel()
 
 
-@pytest.fixture
+@pytest.fixture(scope="session")
 def postgres_target(postgres_config) -> TargetPostgres:
     return TargetPostgres(config=postgres_config)
 
@@ -81,6 +81,7 @@ class AssertionHelper:
     def __init__(self, target: TargetPostgres, metadata_column_prefix: str):
         self.target = target
         self.metadata_column_prefix = metadata_column_prefix
+        self.engine = create_engine(self.target)
 
     def remove_metadata_columns(self, row: dict) -> dict:
         new_row = {}
@@ -107,9 +108,8 @@ def verify_data(
                 table, as determined by lowest primary_key value, or else a list of
                 dictionaries representing every row in the table.
         """
-        engine = create_engine(self.target)
         full_table_name = f"{self.target.config['default_target_schema']}.{table_name}"
-        with engine.connect() as connection:
+        with self.engine.connect() as connection:
             if primary_key is not None and check_data is not None:
                 if isinstance(check_data, dict):
                     result = connection.execute(
@@ -154,9 +154,8 @@ def verify_schema(
                 it is all about the `type` attribute which is compared.
             metadata_column_prefix: The prefix string for metadata columns. Usually `_sdc`.
         """
-        engine = create_engine(self.target)
         schema = self.target.config["default_target_schema"]
-        with engine.connect() as connection:
+        with self.engine.connect() as connection:
             meta = sqlalchemy.MetaData()
             table = sqlalchemy.Table(
                 table_name, meta, schema=schema, autoload_with=connection
@@ -178,7 +177,7 @@ def verify_schema(
                     )
 
 
-@pytest.fixture
+@pytest.fixture(scope="session")
 def helper(postgres_target) -> AssertionHelper:
     return AssertionHelper(
         target=postgres_target, metadata_column_prefix=METADATA_COLUMN_PREFIX