Use exclude_dirs to skip dirs on included libs

to avoid multiple definition of main if included library contains tests.

Example of .arduino-ci.yaml:

```
unittest:
  libraries:
    - libA
  platforms:
    - uno
  exclude_dirs:
    - libA/test
```

See issue Arduino-CI#168

Author: rosogon

Diff for: lib/arduino_ci/cpp_library.rb

@@ -186,7 +186,8 @@ def cpp_files_unittest
     # @param [Array<String>] aux_libraries
     # @return [Array<Pathname>]
     def cpp_files_libraries(aux_libraries)
-      arduino_library_src_dirs(aux_libraries).map { |d| cpp_files_in(d) }.flatten.uniq
+      l = arduino_library_src_dirs(aux_libraries).map { |d| cpp_files_in(d) }.flatten.uniq
+      l = l.reject { |p| @exclude_dirs.any? { |d| p.dirname.to_s.end_with?(d.to_s) } }
     end
 
     # Returns the Pathnames for all paths to exclude from testing and compilation