ENH: add unit tests for `find_issue_date`

[aburrell]

Description

Unit tests to independently cover pysatSpaceWeather.instruments.methods.swpc.find_issue_date.

Potential impact

• Is the feature related to an existing problem? Only integration tests
• How critical is this feature to your workflow? Medium
• How wide of an impact to you anticipate this enhancement having? Catch any changes
• Would this break any existing functionality? No

Potential solution(s)

Write unit tests

Alternatives

Leave using only integration tests