uucore+timeout: accept signals of any casing

[BenWiederhake]

This makes #6217 available to timeout and all other utils that try to parse signal names in the future (at least env: #6377).

Fixes #6381.

I don't have GNU kill available to check, but I think this fixes another issue in kill that we didn't even notice: signal names in the obsolete kill invocation-format:

$ sleep 999 &
[3] 419718
$ /usr/bin/kill -tErM 419718  # procps-ng 4.0.4
[3]+  Terminated              sleep 999

• If this is indeed what GNU does, I need to add a test to make sure we don't regress
• Otherwise, I need to modify this PR to split signal parsing in two steps.

[BenWiederhake]

Changes since last push:

• clippy fix (shame on me)

[BenWiederhake]

Changes since last push:

• Rebased on current main
• Changed the conceptual definition of UResult::signal_name_is to allow signal names of any casing, too. The only change in the code is to change the tests, going from "expect panic" to "expect success". Note that it already links to signal_by_name_or_value.

[BenWiederhake]

Changes since last push:

• Rebased on current main
• Nothing else, I just want to re-run CI, because there were many broken tests last month.

[BenWiederhake]

CI failures seem unrelated:

• Android can't log into something(?)
• Windows can't install crate profiler_builtins(?)
• MacOS has an unrelated error:

--- TRY 3 STDERR:        coreutils::tests test_uptime::test_uptime_with_fifo ---
thread 'main' panicked at tests/by-util/test_uptime.rs:77:10:
'uptime: couldn't get boot time
' does not contain 'uptime: couldn't get boot time: Illegal seek'

[sylvestre]

sorry for the latency

[BenWiederhake]

Thank you <3