runtests: make cleardir() erase dot files too

Because test cases might use dot files.

Closes #5838

tests/runtests.pl

@@ -2716,7 +2716,7 @@ sub cleardir {
     opendir(DIR, $dir) ||
         return 0; # can't open dir
     while($file = readdir(DIR)) {
-        if(($file !~ /^\./)) {
+        if(($file !~ /^\.(|\.)$/)) {
             unlink("$dir/$file");
             $count++;
         }