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

[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.