unittest compiler list is not inherited

[tomduff]

System

Windows

• OS: Windows 10
• ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x64-mingw32]
• bundle -v: Bundler version 1.16.1
• bundle info arduino_ci: arduino_ci (0.1.9)
• g++ -v: gcc version 7.3.0 (GCC)
• Arduino IDE version: 1.8.5

Linux

• OS: Centos 7
• ruby -v: ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
• bundle -v: Bundler version 1.16.1
• bundle info arduino_ci: arduino_ci (0.1.9)
• g++ -v: gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16)
• Arduino IDE version: 1.8.5

Issue Summary

The unittest compiler list is not inherited from the base file resulting in the following error:
The set of compilers (0) isn't empty... ✗

Currently an explicit entry is required in .arduino-ci.yaml:

    # .arduino-ci.yaml
    unittest:
      platforms:
        - uno
        - due
      compilers:
        - g++

Arduino or Unit Test Code, Illustrating the Problem

arduino-ci/SampleProjects/DoSomething