dd: add testsuite entry for write errors

diff --git a/testsuite/dd/dd-reports-write-errors b/testsuite/dd/dd-reports-write-errors
new file mode 100644
index 0000000..4920600
--- /dev/null
+++ b/testsuite/dd/dd-reports-write-errors
@@ -0,0 +1,2 @@
+busybox dd if="$0" of=/dev/full 2>/dev/null || status=$?
+test $status = 1