Testing | Add unit tests for "ConfigFile.updateNow" method

[jfmdev]

Create units test to verify that the ConfigFile.updateNow method works as expected:

• If the result from fetch is successful, then ConfigFile.parseJSONFile should be called with the file content and result (from updateNow) should be true.
• If the result from fetch is successful and encryption is enabled, then ConfigFile.parseJSONFile should be called with the unencrypted file content, and result should be true.
• If fetch throws an exception, then errorMsg (on the local store) shouldn't be null, and result should be false.
• If fetch returns a response with a 400 status, then errorMsg shouldn't be null, and result should be false.
• If the file is encrypted but encryption is disabled, then errMsg shouldn't be null, and result should be false.
• If the file is encrypted but the encryption's key is incorrect, then errMsg shouldn't be null, and result should be false.

Hints:

• You could create a resources folder (inside test) for define sample JSON files (like one encrypted and other not).
• Mock the fetch function, so on the tests you don't really fetch an external file (and instead you use local files).

Blocked by #58, related to #57