tests/extmod/uhashlib_md5: Add coverage tests for MD5 algorithm.

Based on tests/extmod/uhashlib_sha1.

tests/extmod/uhashlib_md5.py

+try:
+    import uhashlib as hashlib
+except ImportError:
+    try:
+        import hashlib
+    except ImportError:
+        # This is neither uPy, nor cPy, so must be uPy with
+        # uhashlib module disabled.
+        print("SKIP")
+        raise SystemExit
+
+try:
+    hashlib.md5
+except AttributeError:
+    # MD5 is only available on some ports
+    print("SKIP")
+    raise SystemExit
+
+md5 = hashlib.md5(b'hello')
+md5.update(b'world')
+print(md5.digest())