test: tweak lib helpers inject to test "use strict"

[sapphi-red]

Description

In #14094, we changed the test to workaround the esbuild's bug (evanw/esbuild#3322, #14094 (comment)).
This PR reverts that part as that bug is now fixed.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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