Remove sleep for TestGroupEntityIndexEventHandlers unit test #35536

Job	Run time
Check whether tests need to be run based on diff	21s
Verify docs and spelling	28s
Unit test (windows-2022)	15m 7s
Unit test (ubuntu-latest)	7m 54s
Golangci-lint (ubuntu-latest)	9m 5s
Golangci-lint (macos-latest)	6m 12s
Integration test	5m 56s
Check tidy, code generation and manifest	4m 29s
Go benchmark test	5m 12s
Build Antrea and antctl binaries	12m 43s
Build Antrea Windows binaries	2m 12s
	1h 9m 39s

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