Do I have to install Web Component Tester (wct) globally? E.g. npm install -g web-component-tester. Is there any way to install it as a local module? E.g. npm install web-component-tester. If so, how do I execute it?
Yes, you can install the web-component-tester locally with the command you mentioned.
To execute it you can run ./node_modules/web-component-tester/bin/wct.