React student form test enhancement

[utsab]

• Delete the DB_URI test and replace with logging statements to remind developer to set the DB_URI.
• Improve add-student-form test by testing the behavior itself instead the internal details (can we test user clicks instead of calling the open function? Can we test the relevant css changes instead of state changes?)
• submit functionality is not completely tested. Also test the request that is sent on submit. And test whether the component properly handles an error response from server.
• test what it renders. Right now we only test that it renders a div which is not enough.