[PATCH] tests: Fix 198.sysval

[Eric Blake]

In my attempt to avoid test failures on Haiku, I caused test failures
on platforms where sh is noisy when reporting a killed sub-process.

* doc/m4.texi (Sysval): Avoid stderr noise during test.
Fixes: 17011ea76a (tests: Skip signal detection on Haiku)
Fixes: https://lists.gnu.org/archive/html/bug-m4/2021-05/msg00029.html
---
 doc/m4.texi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/m4.texi b/doc/m4.texi
index 247f2be5..3b833b2a 100644
--- a/doc/m4.texi
+++ b/doc/m4.texi
@@ -6756,7 +6756,7 @@ Sysval
 ')m4exit(`77')')dnl
 changequote(`[', `]')
 @result{}
-syscmd([/bin/sh -c 'kill -9 $$'; st=$?;
+syscmd([@{ /bin/sh -c 'kill -9 $$'; @} 2>/dev/null; st=$?;
 test $st = 137 || test $st = 265])
 @result{}
 ifelse(sysval, [0], , [errprint([ skipping: shell does not send signal 9
-- 
2.31.1