Add test case for FOR loop variable reusing an existing one

test/asm/for-loop-variable.asm

@@ -0,0 +1,13 @@
+def x = 4
+for x, x
+  def x *= 2
+  println x
+endr
+println x
+
+def y equ 5
+for y, y
+  def y *= 3
+  println y
+endr
+println y

test/asm/for-loop-variable.err

@@ -0,0 +1,3 @@
+error: for-loop-variable.asm(12):
+    'y' already defined as constant at for-loop-variable.asm(8)
+error: Assembly aborted (1 error)!

test/asm/for-loop-variable.out

@@ -0,0 +1,6 @@
+$0
+$2
+$4
+$6
+$4
+$5