gdb.debuginfod/fetch_src_and_symbols.exp: Fix "notice empty URL" test

The gdb_test_multiple pattern for the "notice empty URL" test in
gdb.debuginfod/fetch_src_and_symbols.exp misses expecting the prompt.
Fix it by using -re -wrap.

Also, by using "confirm off", the message GDB prints if Debuginfod
downloading is available doesn't contain "Enable debuginfod" any
longer.  E.g.:

~~~
  (gdb) file testsuite/outputs/gdb.debuginfod/fetch_src_and_symbols/fetch_src_and_symbols
  Reading symbols from testsuite/outputs/gdb.debuginfod/fetch_src_and_symbols/fetch_src_and_symbols...

  This GDB supports auto-downloading debuginfo from the following URLs:
    <http://localhost:123>
  Enable debuginfod for this session? (y or [n])
~~~

~~~
  (gdb) with confirm off -- file testsuite/outputs/gdb.debuginfod/fetch_src_and_symbols/fetch_src_and_symbols
  Reading symbols from testsuite/outputs/gdb.debuginfod/fetch_src_and_symbols/fetch_src_and_symbols...

  This GDB supports auto-downloading debuginfo from the following URLs:
    <http://127.0.0.1:8000>
    <127.0.0.1:8000>
  Debuginfod has been disabled.
  To make this setting permanent, add 'set debuginfod enabled off' to .gdbinit.
  (No debugging symbols found in testsuite/outputs/gdb.debuginfod/fetch_src_and_symbols/fetch_src_and_symbols)
  (gdb)
~~~

I handled that correctly in the other tests that use test_urls, but
had forgotten to update the "notice empty URL" one.

Change-Id: I00040c83466e1494b3875574eb009c571a1504bf