Add tests for circular dependency

Author: boriel

tests/functional/dim_at_label4.bas

@@ -0,0 +1,7 @@
+REM Error: circular dependency
+
+DIM a at @b
+DIM b at @c
+DIM c at @a
+
+

tests/functional/dim_at_label5.bas

@@ -0,0 +1,4 @@
+REM Error: circular dependency
+
+DIM x at @x
+

tests/functional/dim_at_label6.bas

@@ -0,0 +1,4 @@
+
+DIM a at @b + 1
+DIM b at @c
+DIM c at @a

tests/functional/dim_at_label7.bas

@@ -0,0 +1,4 @@
+
+DIM a at @b + 1
+DIM b at @c
+DIM c at @a