wiggle: expand test suite

sync test: show the dummy executor will trap (rather than panic) when a
future inside it pends.

async test: show that the executor is hooked up to a future that pends
for a trivial amount of time.

this adds tokio to the dev-dependencies of wiggle, it shouldn't end up
increasing the build burden for the project as a whole since its already
a dev-dependency.

Cargo.lock

@@ -3923,6 +3923,7 @@ dependencies = [
  "bitflags",
  "proptest",
  "thiserror",
+ "tokio",
  "tracing",
  "wasmtime",
  "wiggle-macro",