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.

crates/wiggle/Cargo.toml

@@ -27,6 +27,7 @@ maintenance = { status = "actively-developed" }
 wiggle-test = { path = "test-helpers" }
 anyhow = "1"
 proptest = "1.0.0"
+tokio = { version = "1", features = ["rt-multi-thread","time", "macros"] }
 
 [[test]]
 name = "wasmtime_async"