[Test] Refactor `inject_webhook_test.go` according to the Developer Guide #891

Job	Run time
e2e (v1.27.11, long-running-resume,from-volume-resume,median-stop)	14m 7s
e2e (v1.27.11, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	21m 24s
e2e (v1.27.11, file-metrics-collector,pytorchjob-mnist)	12m 28s
e2e (v1.27.11, median-stop-with-json-format,file-metrics-collector-with-json-format)	13m 41s
e2e (v1.28.7, long-running-resume,from-volume-resume,median-stop)	16m 22s
e2e (v1.28.7, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	21m 26s
e2e (v1.28.7, file-metrics-collector,pytorchjob-mnist)	12m 43s
e2e (v1.28.7, median-stop-with-json-format,file-metrics-collector-with-json-format)	12m 20s
e2e (v1.29.2, long-running-resume,from-volume-resume,median-stop)	15m 40s
e2e (v1.29.2, grid,bayesian-optimization,tpe,multivariate-tpe,cma-es,hyperband)	20m 51s
e2e (v1.29.2, file-metrics-collector,pytorchjob-mnist)	12m 42s
e2e (v1.29.2, median-stop-with-json-format,file-metrics-collector-with-json-format)	13m 18s
	3h 7m 2s

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