32-bit warnings in test_circuitpadding.c

Ugh, Jenkins is failing. Time to fix it. Here's an example:

16:52:06 src/test/test_circuitpadding.c:1864:43: error: implicit conversion loses integer precision: 'circpad_time_t' (aka 'unsigned long long') to 'long' [-Werror,-Wshorten-64-to-32]
16:52:06   tt_int_op(client_side->padding_info[0]->padding_scheduled_at_usec,
16:52:06   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~