Move shared Spark and Trainer test helpers to kubeflow.common

Summary

The Spark and Trainer SDKs currently maintain duplicate test utilities, including TestCase, SUCCESS, FAILED, and related helper functionality.

To reduce duplication and keep testing utilities consistent across SDK components, these common helpers should be moved to a shared location under kubeflow.common.

Motivation

Currently both components provide nearly identical implementations:

• kubeflow.spark.test.common
• kubeflow.trainer.test.common

Maintaining duplicate implementations increases maintenance overhead and can lead to inconsistencies over time.

Proposed Changes

• Create a shared test helper module under kubeflow.common.
• Move the common testing utilities into the shared module.
• Update Spark tests to import the shared implementation.
• Update Trainer tests to import the shared implementation.
• Remove duplicate implementations where appropriate.

Acceptance Criteria

• Shared test helpers are available under kubeflow.common.
• Spark and Trainer tests use the shared implementation.
• Existing test behavior remains unchanged.
• All affected test suites continue to pass.

ref : https://github.com/kubeflow/sdk/pull/685#discussion_r3683712958