Refactoring more tests

[carusogabriel]

Continuing to refactory tests, like #1430, I've used:

• assertArrayHasKey and assertArrayNotHasKey instead of isset function;
• assertCount instead of count and iterator_count (PHPUnit handles it)

[codecov-io]

Codecov Report

Merging #1435 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1435      +/-   ##
============================================
- Coverage     92.25%   92.22%   -0.03%     
  Complexity     2525     2525              
============================================
  Files           156      156              
  Lines          6687     6687              
============================================
- Hits           6169     6167       -2     
- Misses          518      520       +2

Impacted Files	Coverage Δ	Complexity Δ
src/DynamoDb/SetValue.php	80% <0%> (-20%)	5% <0%> (ø)
src/ApiGateway/ApiGatewayClient.php	0% <0%> (ø)	3% <0%> (ø)	⬇️
src/Sdk.php	90% <0%> (ø)	14% <0%> (ø)	⬇️

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