Merge branch 'ab/portable-more'

Portability fix.

* ab/portable-more:
  tests: fix non-portable iconv invocation
  tests: fix non-portable "${var:-"str"}" construct
  tests: fix and add lint for non-portable grep --file
  tests: fix version-specific portability issue in Perl JSON
  tests: use shorter labels in chainlint.sed for AIX sed
  tests: fix comment syntax in chainlint.sed for AIX sed
  tests: fix and add lint for non-portable seq
  tests: fix and add lint for non-portable head -c N