Improve specs for add method (branch solutions)

[edimossilva]

Complete the following REQUIRED checkboxes:

• [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. Data types exercise: Update spec files

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1. Because:

• Closes #76

2. This PR:

• Add extra specs to add method

3. Additional Information:

• I am also creating a PR to the branch main

[edimossilva]

@rlmoser99 please review