🐛 Bug description

Tests in a crate that uses web-sys IndexedDB bindings fail to run on macOS:
paritytech/parity-common#202 (comment)

🤔 Expected Behavior

The tests should run as they do on linux.

👟 Steps to reproduce

https://github.com/paritytech/parity-common/pull/202/files#diff-354f30a63fb0907d4ad57269548329e3R46

🌍 Your environment

wasm-pack version: 0.8.1
rustc version: 1.37