tests: simplify treatment of the $LOCALE_FR_UTF8 variable

* tests/df/problematic-chars.sh: Rely on gnulib setting
this to "none" where not usable.
* tests/misc/sleep.sh: Likewise.
* tests/printf/printf-mb.sh: Likewise.
* tests/printf/printf-quote.sh: Likewise.
* tests/sort/sort-debug-keys.sh: Likewise.

tests/printf/printf-mb.sh

@@ -22,9 +22,8 @@ print_ver_ printf
 prog='env printf'
 
 unset LC_ALL
-f=$LOCALE_FR_UTF8
-: ${LOCALE_FR_UTF8=none}
 if test "$LOCALE_FR_UTF8" != "none"; then
+  f=$LOCALE_FR_UTF8
   (
    #valid multi-byte
    LC_ALL=$f $prog '%04x\n' '"á' >>out 2>>err

tests/printf/printf-quote.sh

@@ -35,9 +35,8 @@ EOF
 compare exp out || fail=1
 
 unset LC_ALL
-f=$LOCALE_FR_UTF8
-: ${LOCALE_FR_UTF8=none}
 if test "$LOCALE_FR_UTF8" != "none"; then
+  f=$LOCALE_FR_UTF8
   (
    #printable multi-byte
    LC_ALL=$f $prog '%q\n' 'áḃç' > out