Enable linters for test packages

[saima-s]

Is your feature request related to a problem? Please describe.
As of now linters are disabled from test packages in .golangci.yaml file:

issues:
  exclude-rules:
    - path: '_test.go'
      linters:
        - errcheck                      # Errors may be ignored in tests.
        - unparam                       # Tests might have unused function parameters.
        - lll
        - dupl
        - misspell
        - nestif
        - gci

Describe the solution you'd like
After enabling this linter, fix all kind of linter issues coming up in test packages.

[github-actions]

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