tests: remove unnecessary single quotes in perl hash use: ->{'SYM'}

tests: remove unnecessary single quotes in perl hash use: ->{'SYM'}

Run this command:
  git grep -l "limits->{'" \
    | xargs perl -pi -e "s/limits->{'(.*?)'}/limits->{\$1}/g"
* cfg.mk (sc_prohibit_perl_hash_quotes): New rule to match.
* tests/misc/join: Remove quotes.
* tests/misc/sort: Likewise.
* tests/misc/sort-merge: Likewise.
* tests/misc/test: Likewise.
* tests/misc/unexpand: Likewise.
* tests/misc/uniq: Likewise.