[gdb/testsuite] Update regexp in gdb.debuginfod/fetch_src_and_symbols.exp

[gdb/testsuite] Update regexp in gdb.debuginfod/fetch_src_and_symbols.exp

Since commit 7b80401da00 ("Handle DWARF 5 separate debug sections"), test-case
gdb.debuginfod/fetch_src_and_symbols.exp fails here:
...
(gdb) file fetch_src_and_symbols_alt.o^M
Reading symbols from fetch_src_and_symbols_alt.o...^M
warning: could not find supplementary DWARF file \
  (fetch_src_and_symbols_dwz.o) for fetch_src_and_symbols_alt.o^M
(gdb) FAIL: $exp: no_url: file fetch_src_and_symbols_alt.o
...
because this is expected:
...
(gdb) file fetch_src_and_symbols_alt.o^M
Reading symbols from fetch_src_and_symbols_alt.o...^M
warning: could not find '.gnu_debugaltlink' file for \
  fetch_src_and_symbols_alt.o^M
(gdb) PASS: $exp: no_url: file fetch_src_and_symbols_alt.o
...

Fix this by updating the regexp.

Tested on x86_64-linux.