bugfix(core/testing): override provider only if exists in the scope

[kamilmysliwiec]

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?

[x] 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: #1689

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 1865

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 93.696%

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Totals
Change from base Build 1794:	0.01%
Covered Lines:	2996
Relevant Lines:	3141

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💛 - Coveralls

[lock]

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