Add a test to the coordinator for running a Tributary

Impls a LocalP2p for testing.

Moves rebroadcasting into Tendermint, since it's what knows if a message is
fully valid + original.

Removes TributarySpec::validators() HashMap, as its non-determinism caused
different instances to have different round robin schedules. It was already
prior moved to a Vec for this issue, so I'm unsure why this remnant existed.

Also renames the GH no-std workflow from the prior commit.

.github/workflows/no-std.yml

@@ -1,4 +1,4 @@
-name: Tests
+name: no-std build
 
 on:
   push:
@@ -7,7 +7,7 @@ on:
   pull_request:
 
 jobs:
-  test:
+  build:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3