[1.0.2 -> main] Add new testcase where finality advances on alternate branch with lesser lib

[greg7mdp]

Forwarding PR #782 .

Add a testcase where finalizers cannot vote on a block from a different branch because they are locked on a block from their own branch.

/* -----------------------------------------------------------------------------------------------------
            Finality advancing past block claimed on alternate branch
            =========================================================

Time:        t1      t2      t3      t4      t5      t6      t7   
Blocks:
    B0 <---  B1 <--- B2 <--- B3 <-|- B4 <--- B5
                                  |
                                  \----------------- B6 <--- B7
QC claim:
           Strong          Strong  Strong  Strong  Strong   No QC
             B0              B1      B3      B4      B2     achieved

Vote:                      Strong  Strong   Strong  Weak     -

                                                     ^
                                                     |
                                                 validating those weak votes on b2
                                                 would fail if nodes have received b4 and b5
                                                 which advanced lib to b3

    - Node D is isolated and has not seen B3, B4, and B5
    - it received B3 via push_block, (so it can make it its head and produce a child of B3), but has not
      received votes on b3 (only on b2), so b6 includes a strong QC on b2.
    - when b6 is pushed to A, B and C, finalizers of A, B, and C are unable to vote on it, because they
      are locked on B4,
          -> liveness check fails because: `B6' s core.latest_qc_block_timestamp() <  fsi.lock.timestamp`
             because `B2 timestamp < B4 timestamp`.
          -> safety check fails because `B6` does not extend `B4`.
--------------------------------------------------------------------------------------------------------*/

After voting on B5 (which makes B3 final), the finalizers who voted on B5 are locked on B4, and therefore cannot vote on B6:

• liveness check fails because: B6' s core.latest_qc_block_timestamp() < fsi.lock.timestamp because
  B2 timestamp < B4 timestamp.
• safety check fails because B6 does not extend B4.

As a result, this testcase reproduces the scenario described above, and verifies that we lack votes to form a QC on B6.

Although until issue #778 is addressed, the problem described in issue #778 is the cause of the lack of votes, and decide_vote is not even called.

[ericpassmore]

Note:start
category: Tests
component: Internal
summary: New test case where finality advances on lesser LIB.
Note:end