[patch] Dependents and Dependencies in GreenPlum

Hi Hackers,

We found out that when you are connected to a GreenPlum database and try to
get Dependents and Dependencies of an object the application was returning
a SQL error.

This patch splits the SQL query used to retrieve the Dependents,
Dependencies, and Roles SQL file into multiple versioned files.
Add Unit Tests for each file.
Also added __init__.py files to other test directories to run the tests in
them.
Add ORDER BY into Copy Selection Feature test to ensure the results are
retrieved always in the same order
Renamed the Scenario of the xss_checks_pgadmin_debugger_test and skip it
for versions less than 9.1

Thanks

Joao & Sarah