fix GuavaAsyncDoFn flakiness

[nevillelyh]

No description provided.

[codecov-io]

Codecov Report

Merging #858 into master will decrease coverage by 9.75%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #858      +/-   ##
==========================================
- Coverage   80.74%   70.98%   -9.76%     
==========================================
  Files         103      103              
  Lines        3609     3609              
  Branches      494      494              
==========================================
- Hits         2914     2562     -352     
- Misses        695     1047     +352

Impacted Files	Coverage Δ
...n/scala/com/spotify/scio/bigtable/TableAdmin.scala	0% <0%> (-100%)	⬇️
...scala/com/spotify/scio/bigquery/MockBigQuery.scala	0% <0%> (-95.13%)	⬇️
.../scala/com/spotify/scio/testing/util/ItUtils.scala	0% <0%> (-81.25%)	⬇️
.../com/spotify/scio/testing/util/DefaultBucket.scala	0% <0%> (-78.38%)	⬇️
...ala/com/spotify/scio/bigquery/BigQueryClient.scala	17.37% <0%> (-57.81%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	45% <0%> (-55%)	⬇️
...ala/com/spotify/scio/avro/types/TypeProvider.scala	45.31% <0%> (-42.19%)	⬇️
...main/scala/com/spotify/scio/values/DistCache.scala	58.33% <0%> (-25.01%)	⬇️
...in/scala/com/spotify/scio/bigtable/Mutations.scala	20% <0%> (-20%)	⬇️
...main/scala/com/spotify/scio/bigtable/package.scala	66.12% <0%> (-16.13%)	⬇️
... and 6 more

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