backport: Fix the bug in OptCheck.check() introduced in cb779a71

Use 'elif' instead of 'if' to avoid wrong self.result when
self.state is 'off'.

We need unit-tests for the engine checking the correctness: #79

kconfig_hardened_check/__init__.py

@@ -96,7 +96,7 @@ class OptCheck:
         if self.expected == 'is not off':
             if self.state == 'off':
                 self.result = 'FAIL: is off'
-            if self.state == '0':
+            elif self.state == '0':
                 self.result = 'FAIL: is off, "0"'
             elif self.state is None:
                 self.result = 'FAIL: is off, not found'