chore: Enable all unit test and test coverage

[vakrilov]

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.
• You have signed the [CLA].
• 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?

Only tests in the root of the repo were executed with npm test

What is the new behavior?

• All tests (including nested folders) are executed with npm tests
• Added npm run coverage that will report code-coverage using nyc (a.k.a. "the new istanbuljs")