Migrate tests to TypeScript

[rslawik]

• update .gitignore to use glob patterns to not list all files explicitly,
• add typings for chai and mocha,
• fix license in test files
• switch to node_module paths so test work both in wct and es-dev-server
• needed to add typings for wct (for fixture and sinon)

It looks like git did not detect that .ts is basically the same as .html modulo indentation and types.

This is another step for #273