fix the linter error (prealloc) (backport #131)

[mergify]

Problem:
Make validate reports errors from linter.

 Running validation
Running: golangci-lint run
tests/setup/harvester_mock.go:85:6: Consider preallocating results with capacity 1 (prealloc)
	var results []*extv1.CustomResourceDefinition
	    ^
1 issues:
* prealloc: 1

Solution:
Fix the linter error.

Related Issue:

Test plan:

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This is an automatic backport of pull request #131 done by [Mergify](https://mergify.com).