08 Calculator: Add more test cases for subtract and power

[damon314159]

Because

There was only one test case for each of subtract and power, allowing for test fitting to pass all tests

This PR

• Adds more cases to subtract
• Adds more cases to power

Issue

Closes #493

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. 01_helloWorld: Update test cases
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If this PR includes any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder