Implement carton test command

[MaxDesiatov]

This should build tests, passing the resulting binary to a test runner after that. This could be implemented in a few separate steps:

• use wasmer as a basic CLI test runner;
• use the --enable-test-discovery flag to inject the JSON test observer so that test reports can be parsed (blocked by the lack of libIndexStore.dylib in recent snapshots);
• consider using Node.js as another CLI runner option? Tracked separately in Node.js support as a destination environment #174
• host the test bundle via HTTP on localhost with a browser loading an appropriate entrypoint, running the tests and reporting test results back to the HTTP server;
• make wasmer an optional dependency. Tracked separately in Make Wasmer an optional dependency? #177

There's a question of how the target environment is selected. The wasmer option is what will be implemented first, and probably will be the default, but when browser environments are supported, should carton test launch tests in parallel in all possible environments? What should the appropriate option name be for selecting a specific test runner? --target (as in "the target environment"), --host (as in "the webassembly host"), --environment (probably the most unambiguous), --runner? I'm currently thinking of --environment, but all feedback is welcome as usual.

[thecb4]

@MaxDesiatov a couple of questions on this. When we test. We're running a JS Script that tests against the wasm file. Is there any way to mimic UITests and reference "IDs" or names generated in the HTML? If there is a way for introspection and use by the test harness, then we shouldn't care about a browser. We could use pure swift, and test browsers as a secondary effort.

[MaxDesiatov]

Unfortunately, I don't think that's possible without parsing that HTML and maybe even having a complete DOM environment with WebAssembly support. I reckon that in some way it would be an effort comparable to maintaining our own small browser engine. I may be wrong and I'm happy to consider any viable options though.