Add unit test for the `CsvContentExtractionHelper::getLetterSoundsFromCsvBackup`

[vrudas]

Add unit test for extracting LetterSound content from the CSV files that happens on application launch. By having unit tests for this, a developer will be notified sooner when introducing breaking changes.

• good start will be extraction of CsvContentExtractionHelper::getLetterSoundsFromCsvBackup method into a dedicated Util class
• as an example/reference of desired unit tests take look at code of issue #1396

[nya-elimu]

Relates to https://github.com/elimu-ai/webapp/issues/1281