Revert "t6026-merge-attr: don't fail if sleep exits early"

This reverts commit 734fde2.

The point of the test is that the stray process was still running
when 'git merge' did its thing through its completion, so a failure
to "kill" it means we didn't give a condition to the test to trigger
a possible future breakage.  Appending "|| :" to the "kill" is
sweeping a test-bug under the rug.

Author: gitster

t/t6026-merge-attr.sh

@@ -187,7 +187,7 @@ test_expect_success 'custom merge does not lock index' '
 		sleep 1 &
 		echo $! >sleep.pid
 	EOF
-	test_when_finished "kill \$(cat sleep.pid) || :" &&
+	test_when_finished "kill \$(cat sleep.pid)" &&
 
 	test_write_lines >.gitattributes \
 		"* merge=ours" "text merge=sleep-one-second" &&