Only run specific tests during wasm-pack test

[alexcrichton]

This builds on rustwasm/wasm-bindgen#1465 by ensuring that wasm-pack test --node will now only test node.js tests

[ashleygwilliams]

this looks good to me! can we update the documentation to make sure this is clear to folks?

[alexcrichton]

In a sense this is actually a bugfix to update to what the CLI flags actually mean (when you pass --node only running Node tests), so I think the documentation largely already made sense but I tweaked a bit to make sure this is explicitly mentioned.