fix: fix the execution of unit tests with latest unit-test-runner and nativescript-dev-webpack@rc

[Fatme]

With the latest rc of nativescript-dev-webpack it is not possible to run unit tests as the nativescript-dev-webpack searches for app folder inside runner root folder. As such folder doesn't exist in live version of nativescript-unit-test-runner, test command throws an error.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-dev-webpack/blob/master/CONTRIBUTING.md#testing-locally-by-running-e2e-tests
• Tests for the changes are included.

What is the current behavior?

What is the new behavior?

Fixes/Implements/Closes #[Issue Number].