rafttest: add _breakpoint directive

It is a helper case to attach a debugger to when a problem needs
to be investigated in a longer test file. In such a case, add the
following stanza immediately before the interesting behavior starts:

_breakpoint:
----
ok

and set a breakpoint on the _breakpoint case.

raft/rafttest/interaction_env_handler.go

@@ -30,6 +30,16 @@ func (env *InteractionEnv) Handle(t *testing.T, d datadriven.TestData) string {
 	env.Output.Reset()
 	var err error
 	switch d.Cmd {
+	case "_breakpoint":
+		// This is a helper case to attach a debugger to when a problem needs
+		// to be investigated in a longer test file. In such a case, add the
+		// following stanza immediately before the interesting behavior starts:
+		//
+		// _breakpoint:
+		// ----
+		// ok
+		//
+		// and set a breakpoint on the `case` above.
 	case "add-nodes":
 		// Example:
 		//