Add `import time; time.sleep(2]` after `setmocktime` in the abc_rpc_mocktime test to ensure the `NodeClock::now()` assertion is triggered in case of overflow.
Verify that before this change the test node crashes, with `max_int64 // 10**9 + 1` it also crashed, and with `max_int64 // 10**9` it does not crash.
After this change, the RPC now limits the max value.
`ninja check-functional`