Use uint64_t seeds for conformance tests

Before this patch, the python code generated seeds that were sometimes
larger than the maximum uint64_t value, and the C++ code expected 32-bit
seeds. Also, the seeds for all tensors in an test were the same.

Now, the python code generates uint64_t seeds which are different for
each tensor, the jsonschema validates them, and the C++ code uses
their full 64 bits.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: Ib00cfa73f7344ae8407d0fbdb3a180f62e98de86