[Test] Refactor `inject_webhook_test.go` according to the Developer Guide #1523

Job	Run time
Publish Image (darts-cnn-cifar10-cpu, linux/amd64,linux/arm64, examples/v1beta1/trial-images/dart... / Publish Image	7m 40s
Publish Image (darts-cnn-cifar10-gpu, linux/amd64, examples/v1beta1/trial-images/darts-cnn-cifar1... / Publish Image	5m 19s
Publish Image (enas-cnn-cifar10-cpu, linux/amd64,linux/arm64, examples/v1beta1/trial-images/enas-... / Publish Image	7m 58s
Publish Image (enas-cnn-cifar10-gpu, linux/amd64, examples/v1beta1/trial-images/enas-cnn-cifar10/... / Publish Image	3m 40s
Publish Image (pytorch-mnist-cpu, linux/amd64,linux/arm64, examples/v1beta1/trial-images/pytorch-... / Publish Image	10m 11s
Publish Image (pytorch-mnist-gpu, linux/amd64, examples/v1beta1/trial-images/pytorch-mnist/Docker... / Publish Image	4m 42s
Publish Image (simple-pbt, linux/amd64,linux/arm64, examples/v1beta1/trial-images/simple-pbt/Dock... / Publish Image	1m 19s
Publish Image (tf-mnist-with-summaries, linux/amd64,linux/arm64, examples/v1beta1/trial-images/tf... / Publish Image	5m 43s
	46m 32s

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