[tests] Add tests for `GANTRY_NOTIFICATION_CONDITION` #78

Job	Run time
shellcheck	11s
Run tests (gantry_cleanup_images_spec.sh)	1m 57s
Run tests (gantry_common_options_spec.sh)	1m 25s
Run tests (gantry_filters_spec.sh)	1m 15s
Run tests (gantry_login_spec.sh)	1m 51s
Run tests (gantry_manifest_spec.sh)	1m 39s
Run tests (gantry_notify_spec.sh)	2m 45s
Run tests (gantry_parallel_spec.sh)	2m 5s
Run tests (gantry_rollback_spec.sh)	1m 8s
Run tests (gantry_service_multiple_spec.sh)	1m 35s
Run tests (gantry_service_no_running_tasks_spec.sh)	1m 40s
Run tests (gantry_service_single_spec.sh)	1m 38s
Run tests (gantry_update_options_spec.sh)	1m 29s
	20m 38s

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