Tests for order model and schema

[Jeshan7]

Why?

Coverage was less for order's model as well as for its schema.

This change addresses the need by:

Added test cases to increase the coverage.
In order_test.exs(model)

• Test cases added for delete/1, get/1, get_all/0 , user_order/1 and get_all_with_preloads/1
  Added a test case for checking uniqueness of user_id in order_test.exs(schema)

[delivers #164457764]

Checklist

• I have read CONTRIBUTING.md.
• My code follows the style guidelines of this project.
• I have commented on my code, particularly in hard-to-understand areas.
• My code does not generate any (new) credo and compile-time warnings.
• I have updated the documentation wherever necessary.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #482 into develop will increase coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop    #482      +/-   ##
==========================================
+ Coverage    80.25%   80.7%   +0.45%     
==========================================
  Files          176     176              
  Lines         2218    2218              
==========================================
+ Hits          1780    1790      +10     
+ Misses         438     428      -10

Impacted Files	Coverage Δ
apps/snitch_core/lib/core/data/schema/order.ex	100% <0%> (+13.33%)	⬆️
apps/snitch_core/lib/core/data/model/order.ex	100% <0%> (+42.1%)	⬆️

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