In simple terms, the answer to "Do i need to create the separate tests to check this validation message?" is: **Yes**. 

In general, it is advisable to create separate tests for each test case. 

**Reasons:** 

 - Combining several tests will make the tests more clumsy.

 

 - Unless it is a step in the test, you should not have an assert till
   the final step validation.
 - Debugging the failures will become more complicated, if you keep
   adding multiple validations from different paths to a single test.
   The real complexity comes when you have to debug tens of failures
   from random flaky tests, everyday.