fix: testAppendWhileShutdownSuccess race

[yirutang]

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Fixes #<issue_number_goes_here> ☕️

[codecov]

Codecov Report

Merging #907 (4f786fd) into master (7c1bb12) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #907      +/-   ##
============================================
- Coverage     80.38%   80.21%   -0.17%     
  Complexity     1015     1015              
============================================
  Files            76       76              
  Lines          5576     5570       -6     
  Branches        436      420      -16     
============================================
- Hits           4482     4468      -14     
+ Misses          916      913       -3     
- Partials        178      189      +11     

Impacted Files	Coverage Δ	Complexity Δ
...e/cloud/bigquery/storage/v1beta2/StreamWriter.java	79.59% <ø> (+0.39%)	31.00 <0.00> (ø)
...bigquery/storage/v1alpha2/BigQueryWriteClient.java	74.02% <0.00%> (-6.50%)	38.00% <0.00%> (ø%)
.../bigquery/storage/v1beta2/BigQueryWriteClient.java	70.76% <0.00%> (-4.62%)	33.00% <0.00%> (ø%)
...ud/bigquery/storage/v1/BaseBigQueryReadClient.java	58.53% <0.00%> (-2.44%)	12.00% <0.00%> (ø%)
...gquery/storage/v1beta2/BaseBigQueryReadClient.java	58.53% <0.00%> (-2.44%)	12.00% <0.00%> (ø%)
...ery/storage/v1beta1/BaseBigQueryStorageClient.java	71.18% <0.00%> (-1.70%)	22.00% <0.00%> (ø%)

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[stephaniewang526]

Error:  Failures: 
Error:    StreamWriterTest.testAppendWhileShutdownSuccess:767 Call got exception: java.util.concurrent.ExecutionException: java.lang.IllegalStateException: The append result offset 3 does not match the expected offset 5.

Seem like additional changes are required here.