runtests.pl: verify specified test cases

To better allow arguments like "1 to 9999" without flooding the terminal
with error messages, the given test cases range is now checked and only
test numbers with existing files are actually run.

tests/runtests.pl

@@ -4901,6 +4901,19 @@ if ( $TESTCASES eq "all") {
         $TESTCASES .= " $n";
     }
 }
+else {
+    my $verified="";
+    map {
+        if (-e "$TESTDIR/test$_") {
+            $verified.="$_ ";
+        }
+    } split(" ", $TESTCASES);
+    if($verified eq "") {
+        print "No existing test cases were specified\n";
+        exit;
+    }
+    $TESTCASES = $verified;
+}
 
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 # Start the command line log