Unit test for image encoder and decoder.

[kimichenn]

src/utilities/base64.cpp is used to encode camera images, but there are no direct unit tests for its encoder or decoder.

• Tests should cover empty input and inputs of 1, 2, and 3 bytes, including expected padding.
• Tests should cover binary bytes such as 0x00 and 0xFF, including a round trip through encode and decode.
• The tests do not need camera hardware or image files.
• ninja test passes.