runtests: add 'debug' as a feature a test can require

tests/FILEFORMAT

@@ -176,13 +176,13 @@ idn
 ipv6
 large_file
 libz
-netrc_debug
 NSS
 NTLM
 OpenSSL
 SSL
 socks
 unittest
+debug
 
 as well as each protocol that curl supports.  A protocol only needs to be
 specified if it is different from the server (useful when the server

tests/runtests.pl

@@ -2495,6 +2495,11 @@ sub singletest {
                 next;
             }
         }
+        elsif($f eq "debug") {
+            if($debug_build) {
+                next;
+            }
+        }
         elsif($f eq "large_file") {
             if($large_file) {
                 next;