Updating to web-test-runner from karma-test-runner

[JakobVogelsang]

We had two choice to prepare for the move to test runner. We either have to fix import statements in the test or scope elements tag name as proposed #350.

We have decided to do the latter as this has other advantages for us in the future and is the recommended approach of open-wc. Thas means moving can only be done after #350

However, after trying with mwc components we realized this cannot be done at the moment. So we directly update import statements.

[JakobVogelsang]

We have decided to fix import statements as scoping material web components is not possible at the moment.