fix: `.env` doesn't contain duplicate variables

[kysely]

Description

Fixing dotenv utility so that it doesn't create duplicate env variables in .env.

The first commit is just refactoring of the test code. Easier to review just the second commit.

Motivation and Context

When provider has >1 security schemes of same type (or same-named parameters but that shouldn't happen), env vars are added to .env multiple times.

{
  "name": "hubspot",
  "defaultService": "default",
  "parameters": [],
  "services": [
    {
      "baseUrl": "https://api.hubapi.com/",
      "id": "default"
    }
  ],
  "securitySchemes": [
    {
      "type": "apiKey",
      "name": "private-app-legacy",
      "in": "header",
      "id": "private_apps_legacy"
    },
    {
      "type": "apiKey",
      "name": "private-app",
      "in": "header",
      "id": "private_apps"
    }
  ]
}

creates the following .env

HUBSPOT_API_KEY=
HUBSPOT_API_KEY=

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] I have updated the documentation accordingly. For updating Oclif commands documentation use oclif-dev.
• [ ] I have read the CONTRIBUTION_GUIDE document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.