Make test_iconv portable across iconv implementations

[Copilot]

test_iconv.lua assumed utf8 and utf16 aliases are universally accepted by the underlying iconv library. On Termux (and other implementations with stricter charset naming), those alias conversions fail despite utf-8/utf-16 support.

• Test portability adjustment
  • Disabled the two alias-specific conversion calls (utf8, utf16) that are not consistently supported across iconv implementations.
• Expectation clarified in test source
  • Added inline notes documenting that utf-8/utf-16 are the canonical names and non-hyphenated forms are implementation-dependent aliases.

check_one(termcs, "iso-8859-1", iso88591)
-- "utf-8" and "utf-16" are the official names; "utf8" and "utf16" are aliases
-- that are not supported by every iconv implementation.
-- check_one(termcs, "utf8", utf8)
-- check_one(termcs, "utf16", utf16)
check_one(termcs, "EBCDIC-CP-ES", ebcdic)

• Fixes test_iconv.lua fails on termux due to missing hyphens #23

[Tieske]

@copilot rebase this PR

[Copilot]

@copilot rebase this PR

Updated. I merged master into this branch (commit 7c92698) to bring it up to date; in this environment I can’t publish a true rebase because that requires a force-push.