Fix copying test files

[simonihmig]

At a previous iteration a regression was introduced, that copied the test files into the test-app, but including their full path:

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[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: d2e09f3ebecc09b96f1bbb5dc4652cec2158f9e6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | -------------------- | ----- | | ember-addon-migrator | Patch |

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[simonihmig]

Since your a maintainer now, can you add a changeset before merge?

Sure, done!