test(bigtable): deflake goAway test #20661

Job	Run time
generation-config-check	1m 49s
lint	4m 9s
windows	2m 20s
gapic-libraries-bom	19m 34s
bulk-filter	1m 19s
changes	1m 20s
enforcer	19m 3s
split-units (java-bigtable, 8)	5m 41s
split-dependencies (java-bigtable)	5m 15s
split-units (java-bigtable, 17)	8m 7s
split-units (java-bigtable, 11)	6m 6s
split-units (java-bigtable, 25)	7m 20s
split-units (java-bigtable, 21)	7m 34s
split-clirr (java-bigtable)	2m 56s
units (8)	49s
units (25)	34s
units (11)	30s
units (17)	30s
units (21)	35s
conditional-required-check	3s
	1h 35m 34s

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