Add feature test for correct error message

Author: ernilambar

features/db-search.feature

@@ -1102,5 +1102,13 @@ Feature: Search through the database
     And STDOUT should be empty
     And the return code should be 1
 
+    When I try `wp db search example.com --format=count`
+    Then STDERR should be:
+      """
+      Error: The "count" format can only be used for a single table.
+      """
+    And STDOUT should be empty
+    And the return code should be 1
+
     When I run `wp db search mail.example.com wp_options --format=ids`
     Then STDOUT should not be empty