Move LoadTests gradle tasks to test-suites gradle files

- {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](https://issues.apache.org/jira/browse/BEAM-7953). Original Jira may contain additional context.
Reported by: kasiak.

Start by locating the existing LoadTests Gradle tasks and the `test-suites` directory. Check that load tests work with Python 3.x, verify they do not run automatically, and determine whether the global `LOAD_TEST_ENABLED` variable is still required; done means the tasks are moved and these behaviors are confirmed.

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