Add variable information in the detailed listing of a test

[ekouts]

Continuing the work of #3513, it would be nice also to add some information about test variables in the detailed listing (name, default value and type).

[vkarak]

There is the --describe option that shows all the details of the test as JSON. Isn't this enough?

[vkarak]

Maybe this should be combined with #2676. The only tricky part is to differentiate between the test variables defined in the test itself compared to those defined by ReFrame. Maybe the changes in #3489 can help differentiating, as the the class name that defines a variable is now stored.