Changes to tests

Due to the api/schema endpoint now triggering a workflow that requires the SDK client I moved some of the tests from unittest into the functional tests. At the same time I made some minor modifications to the format of the tests and also combined some of them as they seemed a bit redundant.

• backend/tests/unit/api/test_40_schema_api.py the test_schema_load_endpoint_valid_simple() and test_schema_load_endpoint_idempotent_simple() tests have been combined into one TestLoadSchemaAPI::test_schema_load_endpoint_idempotent_simple()
• backend/tests/unit/api/test_40_schema_api.py the test_schema_load_endpoint_valid_with_generics() and test_schema_load_endpoint_idempotent_with_generics() have been combined into TestLoadSchemaAPI::test_schema_load_endpoint_idempotent_with_generics()
• backend/tests/unit/api/test_40_schema_api.py converted test_schema_load_endpoint_valid_with_extensions into `TestLoadSchemaAPI::test_schema_load_endpoint_valid_with_extensions()``
• With regards to the migrated unittests from test_40_schema_api I also simplified them a bit so that all information they need is contained within the json files being loaded instead of loading various other fixtures first.
• backend/tests/unit/api/test_20_schema_api.py. Migrated test_query_at_previous_schema() into TestPreviousVersions::test_query_at_previous_schema()