[BUG]: TODO(bhenning): Split the validation tests into separate unit tests.

Describe the bug

The validation tests below should be split into separate unit tests. Also, all validation errors should be covered in the tests. Split the validation tests in ‘test_validation’ and cover all errors. The current ‘test_validation’ function contains multiple validation checks for explorations. This makes the test logic complex and difficult to maintain. Not all potential validation errors might be covered in this single test.

URL of the page where the issue is observed.

core/domain/exp_domain_test.py

Steps To Reproduce

Open the core/domain/exp_domain_test.py

Expected Behavior

The validation tests should be split into separate unit tests, also covering all test cases.

Screenshots/Videos

No response

What device are you using?

Desktop

Operating System

Linux

What browsers are you seeing the problem on?

No response

Browser version

No response

Additional context

No response

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