Remove Electron test runner

The Electron test runner is no longer maintained, and we've had difficulty getting it working on certain platforms.

Personally it failed to run on my Linux machine, stalling the tests indefinitely. I got it to work by editing the runner to pass the `--no-sandbox` flag to electron.

Locate the Electron test runner configuration and the command or entry point that invokes it; first inspect how the test suite currently selects this runner. Done means the obsolete Electron runner is removed and the tests no longer depend on it or stall on affected platforms.

Written by the indexing model from the issue text.