uucore: format: num_format: add fmt function tests, and workaround 0e10 printing.

[drinkcat]

uucore: format: num_format: add fmt function tests

All the other tests directly called format_float_* functions,
bypassing the additional logic in fmt (negative numbers, padding,
etc.).

This also tests the parse function in mod.rs, which calls back
into try_from_spec here. This also makes it easier to test a lot
of different format combinations without having to do end-to-end
tests in test_printf.rs.

Also add broken tests for the issues in #7509 and #7510.

uucore: format: Workaround BigDecimal printing bug with 0

This is a bigdecimal issue, see
akubera/bigdecimal-rs#144 .

Also add a few tests, including a disabled one (our workaround
is before the call to format_float_decimal).

[github-actions]

GNU testsuite comparison:

GNU test failed: misc/stdbuf.log. misc/stdbuf.log is passing on 'main'. Maybe you have to rebase?
GNU test failed: timeout/timeout.log. timeout/timeout.log is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: misc/stdbuf.log. misc/stdbuf.log is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/timeout/timeout is no longer failing!

[Copilot]

Pull Request Overview

This PR adds tests for the fmt function in uucore’s number formatting module and introduces a workaround for a BigDecimal printing bug when formatting zero.

• Added tests for fmt covering various cases for floats and integers (scientific, hexadecimal, decimal, signed, and unsigned).
• Introduced a BigDecimal zero workaround in the ExtendedBigDecimal branch to force a 0 value to use scale 0 for printing.
• Updated the spell-checker directive to include "cppreference" in addition to the existing tokens.

Comments suppressed due to low confidence (1)

src/uucore/src/lib/features/format/num_format.rs:249

• Ensure that the workaround for BigDecimal zero formatting properly handles all cases, particularly when force_decimal is enabled. Double-check that returning ExtendedBigDecimal::zero() here does not introduce inconsistencies with the expected output across all formatting functions.

if bd.is_zero() {

[drinkcat]

Rebased to fix conflict.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[drinkcat]

akubera/bigdecimal-rs#144 is fixed, but it doesn't hurt to merge this until the next release. Also, this comes with more tests that are good anyway (at least I think so?)... and this is stacked below #7556.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/usage_vs_getopt (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)