ash: bash compat: "shift $BIGNUM" is equivalent to "shift 1"

diff --git a/shell/ash_test/ash-misc/shift1.right b/shell/ash_test/ash-misc/shift1.right
new file mode 100644
index 0000000..b53453c
--- /dev/null
+++ b/shell/ash_test/ash-misc/shift1.right
@@ -0,0 +1,9 @@
+2 3 4
+0: shift: line 1: Illegal number: -1
+1 2 3 4
+2 3 4
+3 4
+4
+
+1 2 3 4
+1 2 3 4