[3.12] gh-159914: Ignore gdb stderr in test_gdb.test_pretty_print (GH-159916) (GH-160201) (#160650)

gh-159914: Ignore gdb stderr in test_gdb.test_pretty_print (GH-159916)

test_strings() gets gdb.host_charset(), but fails if gdb writes
anything to stderr. For example, it fails if gdb logs the warning:

    "/usr/bin/gdb: warning: Couldn't determine a path for the index
    cache directory."

Even if stdout contains the expected encoding ("UTF-8").

Change test_gdb to only fail if stdout is empty, ignore stderr
otherwise to ignore gdb warnings.
(cherry picked from commit 88d922a)
(cherry picked from commit d9b6c22)

Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people

Lib/test/test_gdb/test_pretty_print.py

@@ -116,7 +116,7 @@ def test_strings(self):
             'python import locale; print(locale.getpreferredencoding())')
 
         encoding = stdout
-        if stderr or not encoding:
+        if not encoding:
             raise RuntimeError(
                 f'unable to determine the Python locale preferred encoding '
                 f'of embedded Python in GDB\n'