Use Cadence testing framework for ExampleToken contract

[m-Peter]

Work towards: onflow/developer-grants#148

Description

Uses the Cadence testing framework to test the ExampleToken smart contract.

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels
• Update the version in package.json when there is a change in the smart contracts

[joshuahannan]

Thank you so much for making these changes! This is way cleaner. I just had a few comments but everything looks pretty good so far. 😃

[joshuahannan]

Thanks for addressing my comments! The changes look really good. Did all the tests pass when you ran make test? I need to see why CI isn't running here for you

[m-Peter]

@joshuahannan Both make test & make ci pass locally, and in the GitHub workflow. However, the branch of the PR is coming from a fork, probably that's why it is not running here, but rather in our fork: https://github.com/Build-Squad/flow-ft/actions/runs/5401591136