Alternative (to BeaconChainHarness) BeaconChain testing API (#1380)

The PR:

* Adds the ability to generate a crucial test scenario that isn't possible with `BeaconChainHarness` (i.e. two blocks occupying the same slot; previously forks necessitated skipping slots):

![image](https://user-images.githubusercontent.com/165678/88195404-4bce3580-cc40-11ea-8c08-b48d2e1d5959.png)

* New testing API: Instead of repeatedly calling add_block(), you generate a sorted `Vec<Slot>` and leave it up to the framework to generate blocks at those slots.
* Jumping backwards to an earlier epoch is a hard error, so that tests necessarily generate blocks in a epoch-by-epoch manner.
* Configures the test logger so that output is printed on the console in case a test fails.  The logger also plays well with `--nocapture`, contrary to the existing testing framework
* Rewrites existing fork pruning tests to use the new API
* Adds a tests that triggers finalization at a non epoch boundary slot
* Renamed `BeaconChainYoke` to `BeaconChainTestingRig` because the former has been too confusing
* Fixed multiple tests (e.g. `block_production_different_shuffling_long`, `delete_blocks_and_states`, `shuffling_compatible_simple_fork`) that relied on a weird (and accidental) feature of the old `BeaconChainHarness` that attestations aren't produced for epochs earlier than the current one, thus masking potential bugs in test cases.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/beacon_chain/Cargo.toml

@@ -11,6 +11,7 @@ participation_metrics = []  # Exposes validator participation metrics to Prometh
 
 [dev-dependencies]
 int_to_bytes = { path = "../../consensus/int_to_bytes" }
+maplit = "1.0.2"
 
 [dependencies]
 eth2_config = { path = "../../common/eth2_config" }
@@ -45,6 +46,7 @@ futures = "0.3.5"
 genesis = { path = "../genesis" }
 integer-sqrt = "0.1.3"
 rand = "0.7.3"
+rand_core = "0.5.1"
 proto_array = { path = "../../consensus/proto_array" }
 lru = "0.5.1"
 tempfile = "3.1.0"