CIF-1019 - Fix unit tests for product component

[cjelger]

• fixed unit test to use proper sling model injection
• changed the tests to use sample JSON response from Magento

[codecov-io]

Codecov Report

Merging #103 into master will increase coverage by 6.8%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master     #103     +/-   ##
===========================================
+ Coverage      71.3%   78.11%   +6.8%     
- Complexity      273      316     +43     
===========================================
  Files            38       38             
  Lines          1307     1307             
  Branches        102      102             
===========================================
+ Hits            932     1021     +89     
+ Misses          309      218     -91     
- Partials         66       68      +2

Flag	Coverage Δ	Complexity Δ
#karma	93.61% <ø> (ø)	0 <ø> (ø)	⬇️
#unittests	73.85% <100%> (+8.68%)	316 <0> (+43)	⬆️

Impacted Files	Coverage Δ	Complexity Δ
...onents/internal/models/v1/product/ProductImpl.java	90% <100%> (+52.66%)	53 <0> (+33)	⬆️
...ternal/models/v1/product/VariantAttributeImpl.java	100% <0%> (+10%)	7% <0%> (+1%)	⬆️
...mponents/internal/models/v1/product/AssetImpl.java	73.68% <0%> (+10.52%)	10% <0%> (+2%)	⬆️
...s/internal/models/v1/product/VariantValueImpl.java	100% <0%> (+14.28%)	5% <0%> (+1%)	⬆️
...onents/internal/models/v1/product/VariantImpl.java	96.66% <0%> (+20%)	19% <0%> (+6%)	⬆️

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