Backport tests from yash-rs

[magicant]

[AI-generated]

This PR backports test cases that were added to the scripted tests of yash-rs since 2026-07-01. The commit message lists the source commits in yash-rs.

What is included

• Test cases for extensions that yash also supports: long option names of built-ins (cd --physical, exit --force, jobs --verbose, ulimit --hard, and so on), variable names that are not portable (export foo-bar=1 and similar cases), case-insensitive trap conditions and the SIG prefix, kill -l operands given as signal names, and more.
• Test cases for the portable option of yash-rs whose behavior yash reproduces in the POSIXly-correct mode. They are placed in posix="true" blocks, or run with --posix in kill-y.tst for consistency with the existing test cases in that file. Their names end with "(POSIX)" instead of mentioning the portable option.
• Fixes to the tests themselves: a missing separator before done in path-y.tst, and a background job left running by bg-p.tst.
• New test files true-y.tst and false-y.tst, named after their counterparts in yash-rs.

Adjustments from the original test cases

• command --path : is replaced with command --identify :, because yash does not have the --path option and would reject it even outside the POSIXly-correct mode.
• Test cases that fail on yash because of intended differences between the two shells, and test cases already covered by existing ones, are omitted. Otherwise the relative order of the original test cases is kept.

Testing

make test passes with no errors (4 test cases skipped, none of them related to this change).

🤖 Generated with Claude Code