pickle: test_pickle.py: Turn into real test, add more cases.

Including a test for arbitrary statement execution.

pickle/test_pickle.py

@@ -2,6 +2,23 @@ import pickle
 import sys
 import io
 
-pickle.dump({1:2}, sys.stdout)
 
-print(pickle.loads("{4:5}"))
+def roundtrip(val):
+    t = pickle.dumps(val)
+    t = pickle.loads(t)
+    assert t == val
+
+
+roundtrip(1)
+roundtrip(1.0)
+roundtrip("str")
+roundtrip(b"bytes")
+roundtrip((1,))
+roundtrip([1, 2])
+roundtrip({1:2, 3: 4})
+
+try:
+    pickle.loads("1; import micropython")
+    assert 0, "SyntaxError expected"
+except SyntaxError:
+    pass