add unit tests for internal packages

[ugwis]

Issue #40 , if available:

Description of changes:

• some fix error handling
• add tests

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[ugwis]

$ go test -v ./...
?   	github.com/awslabs/diagram-as-code/cmd/awsdac	[no test files]
?   	github.com/awslabs/diagram-as-code/internal/ctl	[no test files]
?   	github.com/awslabs/diagram-as-code/internal/font	[no test files]
=== RUN   TestFetchFile
--- PASS: TestFetchFile (0.00s)
=== RUN   TestExtractZipFile
--- PASS: TestExtractZipFile (0.00s)
PASS
ok  	github.com/awslabs/diagram-as-code/internal/cache	(cached)
=== RUN   TestDefinitionStructure_LoadDefinitions
=== RUN   TestDefinitionStructure_LoadDefinitions/Valid_YAML_file
--- PASS: TestDefinitionStructure_LoadDefinitions (0.00s)
    --- PASS: TestDefinitionStructure_LoadDefinitions/Valid_YAML_file (0.00s)
=== RUN   TestDefinition
=== RUN   TestDefinition/ValidDefinition
=== RUN   TestDefinition/NestedDefinition
--- PASS: TestDefinition (0.00s)
    --- PASS: TestDefinition/ValidDefinition (0.00s)
    --- PASS: TestDefinition/NestedDefinition (0.00s)
PASS
ok  	github.com/awslabs/diagram-as-code/internal/definition	(cached)
=== RUN   TestGroupInit
--- PASS: TestGroupInit (0.00s)
=== RUN   TestGroupLoadIcon
--- PASS: TestGroupLoadIcon (0.00s)
=== RUN   TestGroupSetters
--- PASS: TestGroupSetters (0.00s)
=== RUN   TestGroupAddLink
--- PASS: TestGroupAddLink (0.00s)
=== RUN   TestGroupAddChild
--- PASS: TestGroupAddChild (0.00s)
=== RUN   TestGroupScale
time="2024-05-02T14:37:33+09:00" level=info msg="Scale "
--- PASS: TestGroupScale (0.00s)
=== RUN   TestHorizontalStackInit
--- PASS: TestHorizontalStackInit (0.00s)
=== RUN   TestLinkInit
--- PASS: TestLinkInit (0.00s)
=== RUN   TestGetThreeSide
--- PASS: TestGetThreeSide (0.00s)
=== RUN   TestDrawNeighborsDots1
--- PASS: TestDrawNeighborsDots1 (0.00s)
=== RUN   TestDrawNeighborDots2
--- PASS: TestDrawNeighborDots2 (0.00s)
=== RUN   TestResource
--- PASS: TestResource (0.16s)
=== RUN   TestMaxInt
=== RUN   TestMaxInt/positive_numbers
=== RUN   TestMaxInt/negative_numbers
=== RUN   TestMaxInt/equal_numbers
--- PASS: TestMaxInt (0.00s)
    --- PASS: TestMaxInt/positive_numbers (0.00s)
    --- PASS: TestMaxInt/negative_numbers (0.00s)
    --- PASS: TestMaxInt/equal_numbers (0.00s)
=== RUN   TestMinInt
=== RUN   TestMinInt/positive_numbers
=== RUN   TestMinInt/negative_numbers
=== RUN   TestMinInt/equal_numbers
--- PASS: TestMinInt (0.00s)
    --- PASS: TestMinInt/positive_numbers (0.00s)
    --- PASS: TestMinInt/negative_numbers (0.00s)
    --- PASS: TestMinInt/equal_numbers (0.00s)
=== RUN   TestFetchColor
=== RUN   TestFetchColor/opaque_color
=== RUN   TestFetchColor/transparent_color
--- PASS: TestFetchColor (0.00s)
    --- PASS: TestFetchColor/opaque_color (0.00s)
    --- PASS: TestFetchColor/transparent_color (0.00s)
=== RUN   TestBlendColor
=== RUN   TestBlendColor/opaque_colors
=== RUN   TestBlendColor/transparent_colors
--- PASS: TestBlendColor (0.00s)
    --- PASS: TestBlendColor/opaque_colors (0.00s)
    --- PASS: TestBlendColor/transparent_colors (0.00s)
=== RUN   TestCalcPosition
=== RUN   TestCalcPosition/center
=== RUN   TestCalcPosition/invalid_position
--- PASS: TestCalcPosition (0.00s)
    --- PASS: TestCalcPosition/center (0.00s)
    --- PASS: TestCalcPosition/invalid_position (0.00s)
=== RUN   TestVerticalStackInit
--- PASS: TestVerticalStackInit (0.00s)
PASS
ok  	github.com/awslabs/diagram-as-code/internal/types	0.756s