This commit also exposes JSONObject's implementation of stringify to the
public, so that it can be used by test-js without having to go through
the interpreter's environment.
This moves most of the work from run-tests.sh to test-js.cpp. This way,
we have a lot more control over how the test suite runs, as well as how
it outputs. This should result in some cool functionality!
This commit also refactors test-common.js to mimic the jest library.
This should allow tests to be much more expressive :)