tests: Add test for hashlib.sha256 .

tests/extmod/sha256.py

+try:
+    import uhashlib as hashlib
+except ImportError:
+    import hashlib
+
+
+h = hashlib.sha256()
+print(h.digest())
+
+h = hashlib.sha256()
+h.update(b"123")
+print(h.digest())
+
+h = hashlib.sha256()
+h.update(b"abcd" * 1000)
+print(h.digest())
+
+print(hashlib.sha256(b"\xff" * 64).digest())
+
+# TODO: running .digest() several times in row is not supported()
+#h = hashlib.sha256(b'123')
+#print(h.digest())
+#print(h.digest())
+
+# TODO: partial digests are not supported
+#h = hashlib.sha256(b'123')
+#print(h.digest())
+#h.update(b'456')
+#print(h.digest())