tests for the constants directory

[SomtochiUmeh]

tests for files in the constants directory:

• feed_constants.dart
• cow_characteristics_constants.dart

[github-actions]

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[github-actions]

LCOV of commit ede901c during Flutter Testing #22

Summary coverage rate:
  lines......: 84.3% (1680 of 1993 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

[github-actions]

LCOV of commit 01b71e9 during Flutter Testing #23

Summary coverage rate:
  lines......: 84.3% (1680 of 1993 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a