test(gax): fix flakiness in BatcherImplTest.testElementsNotLeaking #4230

Job	Run time
filter	1m 13s
java8-compatibility	6m 6s
sdk-platform-java units (8)	4m 30s
dependencies	3m 32s
unmanaged_dependency_check	9m 18s
sdk-platform-java self-service clients (17)	9m 31s
clirr	3m 3s
sdk-platform-java integration (JDK 24+) (26)	10m 6s
gapic-generator-java-bom	3m 5s
gapic-generator-java (8)	11m 49s
sdk-platform-java units (26)	3m 57s
sdk-platform-java units (17)	4m 17s
sdk-platform-java units (11)	4m 7s
sdk-platform-java units (21)	3m 52s
sdk-platform-java self-service clients (11)	9m 2s
sdk-platform-java integration (21)	10m 15s
sdk-platform-java units (25)	4m 13s
sdk-platform-java integration (JDK 24+) (25)	9m 18s
sdk-platform-java integration (17)	10m 13s
dependency_analyzer_unit_test	1m 29s
sdk-platform-java integration (11)	10m 25s
	2h 13m 21s

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