tests/micropython: Add test for consts that are bignums.

tests/micropython/const_intbig.py

+# test constant optimisation, with consts that are bignums
+
+from micropython import const
+
+# check we can make consts from bignums
+Z1 = const(0xffffffff)
+Z2 = const(0xffffffffffffffff)
+print(hex(Z1), hex(Z2))
+
+# check arithmetic with bignum
+Z3 = const(Z1 + Z2)
+Z4 = const((1 << 100) + Z1)
+print(hex(Z3), hex(Z4))

tests/micropython/const_intbig.py.exp

+0xffffffff 0xffffffffffffffff
+0x100000000fffffffe 0x100000000000000000ffffffff