feat: move shared Spark and Trainer test helpers to kubeflow.common

[ChandelAnushka]

What this PR does / why we need it:

Moves shared test utilities (TestCase, SUCCESS, FAILED, DEFAULT_NAMESPACE,
TIMEOUT, RUNTIME) from kubeflow.trainer.test.common and
kubeflow.spark.test.common into a new kubeflow.common.testing module.

Updates all 15 test files across trainer, spark, optimizer, hub, and common to import
from the shared location. Original modules are kept as thin re-export shims for
backward compatibility. Spark-specific constants (SPARK_CONNECT_*) remain in
kubeflow.spark.test.common.

Which issue(s) this PR fixes:

Fixes #700

Checklist:

• Docs included if any changes are user facing
  • N/A — internal test utilities only, no user-facing changes

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[ChandelAnushka]

/cc @andreyvelich @Goku2099

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[ChandelAnushka]

cc @Goku2099