Rename test_harness::manifest to test_case

I thing `TestCase` is better than manifest -- a manifest is more of a
list of items than a series of steps and checks. Plus it conflicts with
a Cargo manifest.

beacon_node/beacon_chain/test_harness/src/lib.rs

@@ -26,7 +26,7 @@
 //! ```
 
 mod beacon_chain_harness;
-pub mod manifest;
+pub mod test_case;
 mod validator_harness;
 
 pub use self::beacon_chain_harness::BeaconChainHarness;