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/core_generator_noexit.py

@@ -0,0 +1,24 @@
+"""
+categories: Core,Generator
+description: Context manager __exit__() not called in a generator which does not run to completion
+cause: Unknown
+workaround: Unknown
+"""
+class foo(object):
+    def __enter__(self):
+        print('Enter')
+    def __exit__(self, *args):
+        print('Exit')
+
+def bar(x):
+    with foo():
+        while True:
+            x += 1
+            yield x
+
+def func():
+    g = bar(0)
+    for _ in range(3):
+        print(next(g))
+
+func()