tests/micropython: Add tests for const names being replaced in parser.

tests/micropython/const2.py

+# check that consts are not replaced in anything except standalone identifiers
+
+X = const(1)
+Y = const(2)
+Z = const(3)
+
+# import that uses a constant
+import micropython as X
+print(globals()['X'])
+
+# function name that matches a constant
+def X():
+    print('function X', X)
+globals()['X']()
+
+# arguments that match a constant
+def f(X, *Y, **Z):
+    pass
+f(1)
+
+# class name that matches a constant
+class X:
+    def f(self):
+        print('class X', X)
+globals()['X']().f()
+
+# constant within a class
+class A:
+    C1 = const(4)
+    def X(self):
+        print('method X', Y, C1, self.C1)
+A().X()

tests/micropython/const2.py.exp

+<module 'micropython'>
+function X 1
+class X 1
+method X 2 4 4