unittests: a dedicated feature in tests

The test runner script now knows if unittests can run and the unit test
setup file says it is one. I also made runtests.pl deal with no
<command> tag set, so that the description file can get even simpler.

tests/runtests.pl

@@ -2314,6 +2314,11 @@ sub singletest {
                 next;
             }
         }
+        elsif($f eq "unittest") {
+            if($debug_build) {
+                next;
+            }
+        }
         elsif($f eq "large_file") {
             if($large_file) {
                 next;
@@ -2562,6 +2567,10 @@ sub singletest {
         # substitute variables in the command line
         subVariables \$cmd;
     }
+    else {
+        # there was no command given, use something silly
+        $cmd="-";
+    }
     if($curl_debug) {
         unlink($memdump);
     }