Fix termination in Worker test

[akosthekiss]

Function names are case sensitive, onMessage is not the same as onmessage.

Related to #98

[chqrlie]

While I approve this fix for consistency reasons, as onmessage is used in quickjs-libc.c, the spelling seems inconsistent with the rest of the API and at least one reputable source uses onMessage: I personally have never used these extensions, can someone explain the difference?

[akosthekiss]

@chqrlie As far as I know, the Worker API of QuickJS is based on the Web Worker API (https://github.com/bellard/quickjs/blob/master/doc/quickjs.texi#L813). According to the HTML Living Standard (https://html.spec.whatwg.org/#workers), workers have onmessage (all lower case) and postMessage (camel case).