runtests.pl: Don't expect $TESTDIR/DISABLED to exist

If a non-standard $TESTDIR is used the file may not be necessary.

Previously a "missing" file resulted in the warning:
readline() on closed filehandle D at ./runtests.pl line 4940.

tests/runtests.pl

@@ -4865,10 +4865,10 @@ if(!$listonly) {
 }
 
 #######################################################################
-# Fetch all disabled tests
+# Fetch all disabled tests, if there are any
 #
 
-open(D, "<$TESTDIR/DISABLED");
+if(open(D, "<$TESTDIR/DISABLED")) {
     while(<D>) {
         if(/^ *\#/) {
             # allow comments
@@ -4879,6 +4879,7 @@ while(<D>) {
         }
     }
     close(D);
+}
 
 #######################################################################
 # If 'all' tests are requested, find out all test numbers