Fix base32/base64 error for file paths ending in slash

[TheAdamWarlock]

Fixes #12670

Before, base32 and base64 checked Path::exists() before opening the file. That turned the real OS error into No such file or directory. Now the file is opened directly, so paths like file/ correctly report Not a directory.

Added regression tests for base32 and base64.

No GNU coreutils source code was read or copied.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/csplit/csplit-heap is now being skipped but was previously passing.
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.

[RenjiSann]

Thanks !