Correct the `test_split::test_suffixes_exhausted` test case so that it
actually exercises the intended behavior of `split`. Previously, the
test fixture contained 26 bytes. After this commit, the test fixture
contains 27 bytes. When using a suffix width of one, only 26 filenames
should be available when naming chunk files---one for each lowercase
ASCII letter. This commit ensures that the filenames will be exhausted
as intended by the test.
Fix two issues with the filename creation algorithm. First, this
corrects the behavior of the `-a` option. This commit ensures a
failure occurs when the number of chunks exceeds the number of
filenames representable with the specified fixed width:
$ printf "%0.sa" {1..11} | split -d -b 1 -a 1
split: output file suffixes exhausted
Second, this corrects the behavior of the default behavior when `-a`
is not specified on the command line. Previously, it was always
settings the filenames to have length 2 suffixes. This commit corrects
the behavior to follow the algorithm implied by GNU split, where the
filename lengths grow dynamically by two characters once the number of
chunks grows sufficiently large:
$ printf "%0.sa" {1..91} | ./target/debug/coreutils split -d -b 1 \
> && ls x* | tail
x81
x82
x83
x84
x85
x86
x87
x88
x89
x9000
