Overwrite tracked file descriptors when exporting COLORED_STDERR_FDS.

If the list of tracked file descriptors is modified to contain unwanted
values (e.g. when a parent process closes or redirects stderr), this
makes it possible to set the tracked file descriptors back to a given
list.

This problem occurred with startx which somehow closes or redirects
stderr. As COLORED_STDERR_FDS was set in the shell running startx, this
disabled coloring stderr for all processes started in the X11 session.

This is no longer an issue. As COLORED_STDERR_FDS is set in the shell
configuration file, it automatically sets the list to the correct value
when a shell is opened in the terminal emulator once X11 is running.

tests/lib.sh

@@ -40,7 +40,10 @@ fi
 LC_ALL=C
 unset LANGUAGE
 
-# Set default COLORED_STDERR_FDS value.
+# Clear user defined variables.
+unset COLORED_STDERR_FDS
+unset COLORED_STDERR_FORCE_WRITE
+# Set default COLORED_STDERR_PRIVATE_FDS value.
 fds=2,
 
 
@@ -78,9 +81,9 @@ run_test() {
     (
         # Standard setup.
         LD_PRELOAD="$library"
-        COLORED_STDERR_FDS="$fds"
+        COLORED_STDERR_PRIVATE_FDS="$fds"
         export LD_PRELOAD
-        export COLORED_STDERR_FDS
+        export COLORED_STDERR_PRIVATE_FDS
 
         # Change pre/post strings for simpler testing.
         COLORED_STDERR_PRE='>STDERR>'