✨ test(VSecM): increase test coverage from core/env

[muratmirgun]

Increase test coverage from core/env and fix test-coverage makefile

Description

Test coverage up to %15.2 from %13.2

Changes

• Add Test for core/env/secret_test.go write tests: https://github.com/vmware-tanzu/secrets-manager/blob/main/core/env/secret_test.go #896
• Add Test for core/env/sentinel_test.go write tests: https://github.com/vmware-tanzu/secrets-manager/blob/main/core/env/sentinel_test.go #897
• Add Test for core/env/keygen_test.go write tests: https://github.com/vmware-tanzu/secrets-manager/blob/main/core/env/keygen_test.go #895
• Add Test for core/env/backoff_test.go write tests: https://github.com/vmware-tanzu/secrets-manager/blob/main/core/env/backoff_test.go #894
• Fix coverage test makefile code

Test Policy Compliance

• I have added or updated unit tests for my changes.
• I have included integration tests where applicable.
• All new and existing tests pass successfully.

Code Quality

• I have followed the coding standards for this project.
• I have performed a self-review of my code.
• My code is well-commented, particularly in areas that may be difficult
  to understand.

Documentation

• I have made corresponding changes to the documentation (if applicable).
• I have updated any relevant READMEs or wiki pages.

Additional Comments

Include any additional comments or context about the PR here.

Checklist

Before you submit this PR, please make sure:

• You have read the contributing guidelines and
  especially the test policy.
• You have thoroughly tested your changes.
• You have followed all the contributing guidelines for this project.
• You understand and agree that your contributions will be publicly available
  under the project's license.

By submitting this pull request, you confirm that my contribution is made under
the terms of the project's license and that you have the authority to grant
these rights.

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Thank you for your contribution to VMware Secrets Manager
🐢⚡️!

[v0lkan]

Looking good.

Thanks for your contribution 🙏 .

Merging this… I’ll test it on the integration server sometime next week.