[3.15] gh-159914: Ignore gdb stderr in test_gdb.test_pretty_print (GH-159916) (#159939)

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)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: miss-islington

Lib/test/test_gdb/test_pretty_print.py

@@ -125,7 +125,7 @@ def test_strings(self):
             'python import gdb; print(gdb.host_charset())')
 
         encoding = stdout.strip()
-        if stderr or not encoding:
+        if not encoding:
             raise RuntimeError(
                 f'unable to determine the host charset of gdb\n'
                 f'stdout={stdout!r}\n'