[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.

---
 gdb/testsuite/lib/gdb.exp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 653f145c1ce..6b0bc8c0af5 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -5080,6 +5080,12 @@ proc default_gdb_init { test_file_name } {
     # tests.
     setenv TERM "dumb"
 
+    # If DEBUGINFOD_URLS is set, gdb will try to download sources and
+    # debug info for f.i. system libraries.  Prevent this.
+    if { [info exists env(DEBUGINFOD_URLS)] } {
+	unsetenv DEBUGINFOD_URLS
+    }
+
     # Ensure that GDBHISTFILE and GDBHISTSIZE are removed from the
     # environment, we don't want these modifications to the history
     # settings.