fix: show correct messages on tns test init command

[Fatme]

Currently tns test init command always print Place your test files under app/tests. However this is not correct when the project has nsconfig.json file. So this PR replaces the hardcoded app/tests with the correct location to tests folder.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
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• Tests for the changes are included.

What is the current behavior?

app/tests folder is hardcoded even for projects with nsconfig.json file

What is the new behavior?

Respect nsconfig.json file and prints the correct location to tests folder.