selftests/damon/debugfs_rm_non_contexts: hide expected write error messages

A selftest case for DAMON debugfs interface has a test for expected
failure.  To make the test output clean, hide the expected failure error
message.

Link: https://lkml.kernel.org/r/20230110190400.119388-9-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/damon/debugfs_rm_non_contexts.sh

@@ -10,7 +10,7 @@ dmesg -C
 
 for file in "$DBGFS/"*
 do
-	echo "$(basename "$f")" > "$DBGFS/rm_contexts"
+	(echo "$(basename "$f")" > "$DBGFS/rm_contexts") &> /dev/null
 	if dmesg | grep -q BUG
 	then
 		dmesg