How to reproduce:
1. Create a new Unity project or open the attached one
2. Open Test Runner window
3. Select "PlayMode" tab

Expected result: There is an option to enable playmode tests.
Actual result: An option to enable playmode tests is not present in the PlayMode tab of the Test Runner window, as well as the context menu of the Test Runner window.

Reproducible with: 2017.4.8f1, 2017.4.12f1
Not reproducible with: 2017.2.3p4, 2017.4.0f1, 2017.4.7f1

Note: On Unity version 2018.1 and later, an option to enable playmode tests can be found in the context menu of the Test Runner window. Therefore, this issue is present only on Unity 2017.4.

Fixed in 2017.4.14f1