util-lib,tests: rework unbase64 so that we skip over whitespace automatically (#7522)

util-lib,tests: rework unbase64 so that we skip over whitespace automatically (#7522)

Let's optimize things a bit, and instead of having to strip whitespace
first before decoding base64, let's do that implicitly while doing so.
Given that base64 was designed the way it was designed specifically to
be tolerant to whitespace changes, it's a good idea to do this
automatically and implicitly.