Reproduction steps:

1. Create a New Project, make sure `com.unity.ugui` is installed
2. Open Package Manifest in a Text Editor
3. Open Test Runner
4. Add Text 1 from Notes to the Package Manifest
5. Focus the Editor
6. See that no Tests are added
7. Add Text 2 to "testables", from Notes
8. Focus the Editor
9. See that Tests are added

Expected Result: Package Tests are added when making a package testable
Actual Result: Package Tests are only added when making a package testable and adding a dummy package

Reproduced with: 2019.2.0a8, 2018.1.0b7, 2018.3.9f1
Did not reproduce on: 2017.4.23f1

Text 1:
",
"testables": [
"com.unity.ugui"
]"

Text 2:
"com.unity.ads"

Note:
2017.4.23f1 packages aren't fully implemented