fix(testing-sdk): fix checkpoint serialization #326

Job	Run time
setup	5s
Basic Step	57s
Hello World	1m 9s
Step with Name	1m 5s
Callback	58s
Multiple Wait	56s
Map Operations	1m 12s
Run in Child Context	58s
Wait State	1m 12s
Parallel Operations	1m 10s
Block Example	54s
Wait for Condition	54s
Step with Retry	1m 4s
Simple Execution	58s
Steps with Retry	1m 6s
Logger Example	1m 12s
Run in Child Context Large Data	1m 7s
Map with Min Successful	59s
Map with Failure Tolerance	1m 7s
Map with Max Concurrency	1m 9s
Parallel with Wait	1m 0s
Parallel with Max Concurrency	1m 12s
Parallel with Batch SerDes	1m 15s
Parallel with Failure Tolerance	1m 5s
Map with Custom SerDes	1m 11s
Map with Batch SerDes	1m 1s
Parallel with Custom SerDes	1m 10s
	28m 6s

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