add a dumb timeout logic

[yamt]

issue: https://github.com/WebAssembly/wasi-testsuite/issues/42

a few notes on this implementation:

• the timeout is hardcoded to 3 seconds for now. we can add a configuration in per-test json files when necessary.
• unit test doesn't cover the new code. i have no plan to write tests at this point.
• adapters may need something like https://github.com/yamt/toywasm/pull/117 for better cleanup

[yamt]

@loganek please review when you have time. this is not urgent for me.

[loganek]

Thanks for the change, and sorry for being late with the review. I'll try to get to it by end of this week.

[yamt]

@loganek ping