twister: improve diagnostics for 'any' set/list types

Since pykwalify does not support multi-type fields, we are forced to use
the 'any' type. This is used for fields that allow sets/lists written as
either YAML lists or space-separated strings. This patch checks for
str/list Python type, and raises a configuration error if not true so
that user obtains better error messages in case the configuration file
contains invalid format.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>