Exercise: Add tests

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1. Add some tests to the 01-add.t file.
2. Create a new file called 02-multiply.t and add tests to verify the multiply() function.
3. See how it works when running directly with perl and using harness.
4. Increase the version number in the module.
5. Add a new entry to the Changes file describing your changes.
6. Create the release of of new CPAN distribution.