Move LoadTests gradle tasks to test-suites gradle files

[damccorm]

• {color:#172b4d}Ensure that load tests works with Python 3.x{color}
• {color:#172b4d}Move gradle task to test-suites directory{color}
• {color:#172b4d}Check if they don't run automatically{color}
• {color:#172b4d}Check if global variable LOAD_TEST_ENABLED is required{color}

Imported from Jira BEAM-7953. Original Jira may contain additional context.
Reported by: kasiak.