test(@nestjs/config) add more tests for priority

[bejewel-kyoungmin]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

I have a curiosity to see how config module works.
I think nestjs documents don't provide enough explanations.
If both .env file and load configration are provided, load configuration override .env vars?
If multiple load configrations are provided, last configuration take precedence?
That's why I add a couple of tests.

These tests answer following questions.
both .env file and load configuration, what's the priority?
multiple load configurations, what's the priority?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm