test whiteboard: convert it to a test instance first class attribute

Based on some discussions, we decided to make the whiteboard a test
instance attribute (string) that is automatically exposed to test
results.

For the use case of persisting multiple files with the test result,
the test data dir can still be used (refer to the `gendata` test).

Signed-off-by: Cleber Rosa <crosa@redhat.com>
whiteboard test: adapt test to new whiteboard implementation

Signed-off-by: Cleber Rosa <crosa@redhat.com>
plugins/jsonresult.py: include test whiteboard

Signed-off-by: Cleber Rosa <crosa@redhat.com>