Move mojo/apps/js tests et al to mojo/edk/js/test,tests

The apps_js_unitttests test executable is now called js_integration_tests.

The connection_unittests are now included in the JS unittests. To enable that the threading module, which just provides a quit method, was added to mojo/public/js. Its implementation is mojo/edk/js.

Integration tests are in the edk/js/tests directory and test support code is in edk/js/test.

BUG=430520
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/709603002