Use Cadence testing framework for `ExampleToken` contract

[m-Peter]

Work towards: https://github.com/onflow/developer-grants/issues/148

Description

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

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For contributor use:

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

[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