test(spanner): add timeouts to AsyncResultSetImplTest #17791

Job	Run time
gapic-bom	1m 46s
owlbot-py	1m 15s
owlbot-yaml	1m 3s
root-pom	1m 39s
generate-readme	1m 21s
package_name_check	1m 2s
gitignore_correctness	1m 2s
group_id_check_for_maps_libraries	7m 54s
	17m 2s

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