tests/cpydiff: Add initial set of tests for uPy-CPython differences.

These tests are intended to fail, as they provide a programatic record of
differences between uPy and CPython.  They also contain a special comment
at the start of the file which has meta-data describing the difference,
including known causes and known workarounds.

tests/cpydiff/modules_array_subscrstep.py

@@ -0,0 +1,9 @@
+"""
+categories: Modules,array
+description: Subscript with step != 1 is not yet implemented
+cause: Unknown
+workaround: Unknown
+"""
+import array
+a = array.array('b', (1, 2, 3))
+print(a[3:2:2])