generate_expected_output.py: suppress annoying warning about non-UTF8

On my system, en_US.UTF-8 is apparently not properly detected, causing
the warning about running on a non-UTF8 locale to be printed. I don't
understand why because strace shows that the proper locale file was
opened:

openat(AT_FDCWD, "/usr/lib/locale/en_US.utf8/LC_CTYPE", O_RDONLY|O_CLOEXEC) = 3

And it doesn't get printed when running the tests manually, only when
running with the python script.

Detected locale "C" with character encoding "ANSI_X3.4-1968", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.

Change-Id: I0ea60c82852bc3039952fffdd9136ed4ada43158
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/testlib/selftests/generate_expected_output.py

@@ -239,7 +239,7 @@ def baseEnv(platname=None,
             keep += preserveLib
 
         cached = dict(
-            LC_ALL = 'en-US.UTF-8', # Use standard locale
+            LC_ALL = 'C.UTF-8', # Use standard locale
             # Avoid interference from any qtlogging.ini files, e.g. in
             # /etc/xdg/QtProject/, (must match tst_selftests.cpp's
             # processEnvironment()'s value):