[test] Test tail-call to imported function

[keithw]

There aren't any tests yet that use imports or attempt to tail-call an imported function -- there probably should be one (this would have caught a bug in wasm2c, WebAssembly/wabt#2420).

[rossberg]

Good point! Did you produce some that you could contribute?

[rossberg]

Closing via #25 / upstream PR.