Refactor engine client test_create_job test names for better clarity and consistency (#6885)

Split initial `test_create_job` test into:
- Four successful tests:
  - `test_create_job_with_all_parameters`
  - `test_create_job_without_labels`
  - `test_create_job_without_description`
  - `test_create_job_without_specified_job_id`
- One failure test:
  - `test_create_job_with_invalid_priority`

*Note: Splitting successful tests to stay consistent with existing
tests:*
-
*`test_create_job_with_invalid_processor_and_device_config_arguments_throws`*
  - *`test_create_job_with_run_name_and_device_config_name_succeeds`*

EDIT: Also refactored the test cases to use pytest fixtures for
`default_run_context` and `default_engine_client` for consistency and to
reduce redundancy.

Fixes #6761

Author: ayshiff