After D14912, the abc_rpc_mocktime test occasionnaly triggers an assertion error in time.cpp on slow machines
AssertionError: Unexpected stderr bitcoind: /home/pierre/dev/bitcoin-abc/src/util/time.cpp:78: static NodeClock::time_point NodeClock::now(): Assertion `ret > 0s' failed. !=
This is because NodeClock::Now() returns a time with nanoseconds precision (encoded as an int64 internally), no matter if the time is mocked or not, and the test tries to set a mocktime of max_int64 seconds.
Fix the RPC to not accept values that would cause an overflow.