Add shift assembly test

test/asm/shifts.s

+.text
+  li $a0, 6
+  sll $a0, $a0, 2
+  li $v0, 1
+  syscall
+
+  srl $a0, $a0, 3
+  li $v0, 1
+  syscall
+
+  # Exit
+  li $v0, 10
+  syscall
+.data