[gdb/testsuite] Fix port detection in gdb.debuginfod/fetch_src_and_symbols.exp

On OBS I ran into this failure with test-case
gdb.debuginfod/fetch_src_and_symbols.exp:
...
Failed to listen for connections: Address already in use^M
[Thu Oct 21 11:48:49 2021] (559/559): started http server on IPv6 port=8000^M
  ...
FAIL: gdb.debuginfod/fetch_src_and_symbols.exp: local_url: find port timeout
...

The test-case is trying to start debuginfod on a port to see if it's
available, and it handles either this message:
  "started http server on IPv4 IPv6 port=$port"
meaning success, or:
  "failed to bind to port"
meaning failure, in which case the debuginfod instance is killed, and we try
the next port.

The test-case only uses the v4 address 127.0.0.1, so fix this by:
- accepting "started http server on IPv4 port=$port"
- rejecting "started http server on IPv6 port=$port"

Tested on x86_64-linux.

gdb/testsuite/gdb.debuginfod/fetch_src_and_symbols.exp

@@ -180,12 +180,17 @@ proc local_url { } {
 	spawn debuginfod -vvvv -d $db -p $port -F $debugdir
 	expect {
 	    "started http server on IPv4 IPv6 port=$port" { set found 1 }
-	    "failed to bind to port" { kill_wait_spawned_process $spawn_id }
+	    "started http server on IPv4 port=$port" { set found 1 }
+	    "started http server on IPv6 port=$port" {}
+	    "failed to bind to port" {}
 	    timeout {
 		fail "find port timeout"
 		return -1
 	    }
 	}
+	if { ! $found } {
+	    kill_wait_spawned_process $spawn_id
+	}
     }
 
     set metrics [list "ready 1" \