authority_tests.rs failing sometimes

[lxfind]

Seeing this failure, but not always reproducible:

failures:

---- authority::authority_tests::test_get_latest_parent_entry stdout ----
thread 'authority::authority_tests::test_get_latest_parent_entry' panicked at 'attempt to add with overflow', fastpay_core/src/unit_tests/authority_tests.rs:1166:5
stack backtrace:
   0: rust_begin_unwind
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/panicking.rs:107:14
   2: core::panicking::panic
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/panicking.rs:48:5
   3: fastpay_core::authority::authority_tests::test_get_latest_parent_entry::{{closure}}
             at ./src/unit_tests/authority_tests.rs:1166:5
   4: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/future/mod.rs:80:19
   5: <core::pin::Pin<P> as core::future::future::Future>::poll
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/core/src/future/future.rs:119:9
   6: tokio::runtime::basic_scheduler::CoreGuard::block_on::{{closure}}::{{closure}}::{{closure}}
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/runtime/basic_scheduler.rs:516:48
   7: tokio::coop::with_budget::{{closure}}
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/coop.rs:102:9
   8: std::thread::local::LocalKey<T>::try_with
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/std/src/thread/local.rs:399:16
   9: std::thread::local::LocalKey<T>::with
             at /rustc/db9d1b20bba1968c1ec1fc49616d4742c1725b4b/library/std/src/thread/local.rs:375:9
  10: tokio::coop::with_budget
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/coop.rs:95:5
  11: tokio::coop::budget
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/coop.rs:72:5
  12: tokio::runtime::basic_scheduler::CoreGuard::block_on::{{closure}}::{{closure}}
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/runtime/basic_scheduler.rs:516:25
  13: tokio::runtime::basic_scheduler::Context::enter
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/runtime/basic_scheduler.rs:374:19
  14: tokio::runtime::basic_scheduler::CoreGuard::block_on::{{closure}}
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/runtime/basic_scheduler.rs:515:36
  15: tokio::runtime::basic_scheduler::CoreGuard::enter::{{closure}}
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/runtime/basic_scheduler.rs:582:57
  16: tokio::macros::scoped_tls::ScopedKey<T>::set
             at /Users/xun/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.16.1/src/macros/scoped_tls.rs:61:9
...

It's panicing on this line:

    x[last_index] += 1;