[tests] Add tests for `GANTRY_NOTIFICATION_CONDITION` #79

Job	Run time
Run tests (gantry_cleanup_images_spec.sh)	1m 55s
shellcheck	10s
Run tests (gantry_common_options_spec.sh)	1m 29s
Run tests (gantry_filters_spec.sh)	1m 9s
Run tests (gantry_login_spec.sh)	1m 55s
Run tests (gantry_manifest_spec.sh)	1m 46s
Run tests (gantry_notify_spec.sh)	2m 28s
Run tests (gantry_parallel_spec.sh)	2m 15s
Run tests (gantry_rollback_spec.sh)	1m 11s
Run tests (gantry_service_multiple_spec.sh)	1m 45s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 38s
Run tests (gantry_service_single_spec.sh)	1m 35s
Run tests (gantry_update_options_spec.sh)	1m 34s
	20m 50s

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