Set default test timeout to 60s

[gitmalong]

cargo's default test timeout is at 60s. For consistency I suggest to set it for wasm-pack executed tests to 60s instead of 20s.

[daxpedda]

This is wasm-bindgen not wasm-pack.

Happy to change it to follow Cargo's default though, any PR's welcome.
We should probably also document #2036 at this point.