tests/extmod/uctypes_sizeof_layout: Test for sizeof of different layout.

On almost all realistic platforms, native layout should be larger (or
equal) than packed layout.

tests/extmod/uctypes_sizeof_layout.py

+try:
+    import uctypes
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+desc = {
+    "f1": 0 | uctypes.UINT32,
+    "f2": 4 | uctypes.UINT8,
+}
+
+
+# uctypes.NATIVE is default
+print(uctypes.sizeof(desc) == uctypes.sizeof(desc, uctypes.NATIVE))
+
+# Here we assume that that we run on a platform with convential ABI
+# (which rounds up structure size based on max alignment). For platforms
+# where that doesn't hold, this tests should be just disabled in the runner.
+print(uctypes.sizeof(desc, uctypes.NATIVE) > uctypes.sizeof(desc, uctypes.LITTLE_ENDIAN))
+
+# When taking sizeof of instantiated structure, layout type param
+# is prohibited (because structure already has its layout type).
+s = uctypes.struct(0, desc, uctypes.LITTLE_ENDIAN)
+try:
+    uctypes.sizeof(s, uctypes.LITTLE_ENDIAN)
+except TypeError:
+    print("TypeError")

tests/extmod/uctypes_sizeof_layout.py.exp

+True
+True
+TypeError