base32, base64, baseenc:Simplifying the base encoding uu_app and adding basic buffer tests

[ChrisDryden]

I'm working on systematically going through the CodeCoverage to see what areas have gaps to add testing for. I saw that for the base encoding bins the uu_app did not have coverage and it was because it was following a format where the code to create the command was being duplicated. I did a bit of a refactor so that the code to create the uu_app can follow a similar format of all of the other bins. This also has the side effect of increasing the code coverage.

I also noticed that the code that tests the buffering when encoding did not have a test to cover that use case, so I added a very basic one to go through that code path and validate that the output is still being converted correctly.

[ChrisDryden]

I see the Github actions tests failed on an infrastructure issue after adding that spellcheck flag, I'm guessing its a bit flaky

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/overlay-headers (passes in this run but fails in the 'main' branch)

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)

[cakebaker]

Thanks!