[gdb/testsuite] Ignore DEBUGINFOD_URLS

[gdb/testsuite] Ignore DEBUGINFOD_URLS

On openSUSE Tumbleweed, DEBUGINFOD_URLS is now defined by default:
...
$ echo $DEBUGINFOD_URLS
https://debuginfod.opensuse.org/
...

With DEBUGINFOD_URLS defined we run into:
...
FAIL: gdb.mi/mi-sym-info.exp: List all functions from debug information only \
  (timeout)
...
as reported in PR27667.

There's a latency of ~0.5s per request, which is ok-ish for interactive usage.
But the symbol-info-functions command ends up issuing 21 source requests,
which means we easily run into the 10s timeout.

Fix this by unsetting DEBUGINFOD_URLS in default_gdb_init.

gdb/testsuite/ChangeLog:

2021-03-31  Tom de Vries  <tdevries@suse.de>

	PR testsuite/27667
	* lib/gdb.exp (default_gdb_init): Unset DEBUGINFOD_URLS.