add validation for regex flags

Author: miya0001

features/db-search.feature

@@ -701,6 +701,9 @@ Feature: Search through the database
     When I run `wp db search 'unfindable' --regex`
     Then STDOUT should be empty
 
+    When I run `wp db search 'unfindable' --regex --regex-flags='abcd'`
+    Then the return code should be 1
+
     When I run `wp db search '[0-9é]+?https:' --regex --regex-flags=u --before_context=0 --after_context=0`
     Then STDOUT should contain:
       """