test: test ux improvements

[owenpearson]

Includes a couple of improvements for the tests in this project:

• by default, outputs debug logs for failing tests and no logs for passing tests, making the test results much easier to read and allowing us to see full client logs for test flakes in CI (especially helpful if we aren't able to reproduce the flake locally). you can still configure all of the tests to emit logs to stdout by explicitly setting a log level (using ABLY_LOG_LEVEL in nodejs or the log_level query param in browser).
• use source map for node tests (no more stack traces pointing to 'ably-node:8392')
• uses the mocha CLI to run node tests, avoiding the grunt-mocha task. this makes it easier to leverage mocha's CLI options and means that the output works with colorterm, see screenshots:

example test stdout with colorterm:

example test failure output:

[VeskeR]

@VeskeR I've added 61e28de to make the web_server executable a module

Thank you!
And removal of kexec is a nice side effect of this change, that package was not compatible with Windows at all.