Add a test for validation of rules

Signed-off-by: akugarg <akanksha.garg2k@gmail.com>

Author: akugarg

src/licensedcode/models.py

@@ -918,6 +918,11 @@ def validate(self, licensing=None):
         if not (0 <= self.minimum_coverage <= 100):
             yield 'Invalid rule minimum_coverage. Should be between 0 and 100.'
 
+        if self.is_unknown:
+            if not "unknown" in license_expression:
+                yield 'is_unknown rule cannot be true if license does not fall in unknown category.'
+
+
         if not is_false_positive:
             if not (0 <= self.relevance <= 100):
                 yield 'Invalid rule relevance. Should be between 0 and 100.'