factor test failures

[nbraud]

The test I added in #1831 is sometimes exhibiting an invariant violation:

---- factor::tests::exponentiate_factors stdout ----
thread 'factor::tests::exponentiate_factors' panicked at 'assertion failed: `(left == right)`
  left: `Some((18451, 2))`,
 right: `None`', src/uu/factor/src/factor.rs:33:9
stack backtrace:
   0: rust_begin_unwind
   1: std::panicking::begin_panic_fmt
   2: uu_factor::factor::Decomposition::add
             at ./src/factor.rs:33:9
   3: uu_factor::factor::factor
             at ./src/factor.rs:209:25
   4: uu_factor::factor::tests::exponentiate_factors::prop
             at ./src/factor.rs:282:28
   5: <fn(A,B) .> T as quickcheck::tester::Testable>::result::{{closure}}
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:384:35
   6: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /build/rustc-1.49.0-src/library/std/src/panic.rs:322:9
   7: std::panicking::try::do_call
             at /build/rustc-1.49.0-src/library/std/src/panicking.rs:381:40
   8: __rust_try
   9: std::panicking::try
             at /build/rustc-1.49.0-src/library/std/src/panicking.rs:345:19
  10: std::panic::catch_unwind
             at /build/rustc-1.49.0-src/library/std/src/panic.rs:396:14
  11: quickcheck::tester::safe
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:415:5
  12: <fn(A,B) .> T as quickcheck::tester::Testable>::result
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:384:21
  13: quickcheck::tester::QuickCheck<G>::quicktest
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:134:19
  14: quickcheck::tester::QuickCheck<G>::quickcheck
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:176:36
  15: quickcheck::tester::quickcheck
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:196:5
  16: uu_factor::factor::tests::exponentiate_factors
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/lib.rs:65:21
  17: uu_factor::factor::tests::exponentiate_factors::{{closure}}
             at /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/lib.rs:61:17
  18: core::ops::function::FnOnce::call_once
             at /build/rustc-1.49.0-src/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread 'factor::tests::exponentiate_factors' panicked at '[quickcheck] TEST FAILED (runtime error). Arguments: (Factors(RefCell { value: Decomposition([(18451, 1), (3, 1)]) }), 3)
Error: "assertion failed: `(left == right)`\n  left: `Some((18451, 2))`,\n right: `None`"', /home/nicoo/.cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:178:28

I believe the bug was introduced when prototype PR #1571 was merged, or by one of the subsequent bug fixes.
Thanks a bunch @sylvestre for sending me the test failure you encountered.

[sylvestre]

I also see it

---- factor::tests::exponentiate_factors stdout ----
thread 'factor::tests::exponentiate_factors' panicked at 'assertion failed: `(left == right)`
  left: `Some((8237, 2))`,
 right: `None`', src/uu/factor/src/factor.rs:33:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'factor::tests::exponentiate_factors' panicked at '[quickcheck] TEST FAILED (runtime error). Arguments: (Factors(RefCell { value: Decomposition([(8237, 1), (17, 1), (5, 1)]) }), 3)
Error: "assertion failed: `(left == right)`\n  left: `Some((8237, 2))`,\n right: `None`"', /cargo/registry/src/github.com-1ecc6299db9ec823/quickcheck-0.9.2/src/tester.rs:178:28


failures:
    factor::tests::exponentiate_factors

https://github.com/uutils/coreutils/runs/2154792917