[pgadmin4][patch] Unit test fail on GreenPlum (#3190)

Hello Hackers,

Attached you can find the patch that skip some tests and correct issues on
SQL that are failing when trying to connect to a GreenPlum database.

We did this by adding a attribute to to test_json called "db_type" that
will carry the type of database we are running tests against.

When we run tests against a GreenPlum instance the configuration would look
like this:

{
"name": "GreenPlum",
"comment": "GreenPlum DB",
"db_username": "gp",
"host": "localhost",
"db_password": "",
"db_port": 5433,
"maintenance_db": "postgres",
"sslmode": "prefer",
"tablespace_path": "",
"enabled": true,
"db_type": "gpdb"
}

Thanks
Joao