IF: add more flexibility for IF transition to Boost libtester

[linh2931]

Currently set_finalizers of libtester takes only a vector of producer names for IF transision. set_finalizer hardcodes the threshold and weights for each producer and sets up all producers to vote in the node.

To facilitate more complex tests, set_finalizer or another function like transition_to_if should take threshold, weights, and voting producers as parameters to set up IF testing.

Tests using the new function will be added in the same PR resolving the issue.

[arhag]

Libtester should have functionality to allow for tests with explicit control of votes: how they vote (strong/weak) via manipulation of finalizer safety information, where the votes arrive (which nodes or instances of libtester), and when the votes arrive.

[linh2931]

Manipulating finalizer safety info is now in #2233.

[greg7mdp]

weak vote tests are not passing and were disabled. Reopening until these tests can be re-enabled.

[linh2931]

After the finality safety info feature is in, we cannot simply change vote message to simulate weak votes, as now signatures are different for strong and weak votes. Need to manipulate finality safety info.