tests: yaml: add test for generator expressions

Add tests to the YAML suite for generator expressions. The test checks
that generator expressions are handled as expected at different stages:

 - immediately after yaml_save(), the file must contain only non-genex
   entries;
 - at a later time, after the target has been built, the file must
   contain all the expanded genex values.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>