Support auto disabling of unit tests with an optional enabled script

Restore the unicode test case using that.

test/compose/unicode/cmd

@@ -0,0 +1 @@
+~

test/compose/unicode/enabled

@@ -0,0 +1,2 @@
+#!/bin/sh
+locale | grep LC_CTYPE | grep 'utf8\|UTF-8' > /dev/null

test/compose/unicode/in

@@ -0,0 +1 @@
+%(maïs mélange bientôt)

test/compose/unicode/out

@@ -0,0 +1 @@
+MAÏS MÉLANGE BIENTÔT

test/run

@@ -18,6 +18,8 @@ main() {
     name=$(basename $PWD)
     if ! test -e cmd; then
       echo "$indent$name"
+    elif test -x enabled && ! ./enabled; then
+      echo "$indent$name (disabled)" | colorize yellow normal
     else
       number_tests=$(($number_tests + 1))
       touch in; cp in out