After PR #787 lands, the js and nosync packages will be considered "core" and embedded into the GopherJS compiler via compiler/gopherjspkg package. gopherjspkg needs to be regenerated whenever js or nosync changes (not often).

To make everything work, I embedded the entire js, nosync packages, including their tests, and did c964bad to make their tests run.

When reviewing #779, I've realized we might want to modify/add tests in js package more often. Having to regenerate gopherjspkg is inconvenient (e.g., especially so when in the process of writing/modifying a test). It also increases the size of the compiler, and I don't want to feel pressure to keep the tests smaller because of that.

Finally, there's always been an odd split of tests in js package and tests in tests package. Sometimes it's not clear where to add a test.

By moving js tests from js package into tests, all these problems are resolved.

All of js tests are external, so I think its should be easy. I think it's better than trying to make js normal package embedded but its tests not embedded.

I think a good way of thinking github.com/gopherjs/gopherjs/js package (after #787) is that it's somewhat like C pseudo-package (related to cgo), and it wouldn't make sense to try to put tests inside C.