Fixes main window test delay

[Atomic-Germ]

Right now, the first test in tests/main-window takes a stupidly long time. This patch fixes that, but I'm not entirely sure of why it fixes it.

What change is being introduced by this PR?

• adds await window.waitForLoadState('domcontentloaded') after const window = await electronApp.firstWindow();.

How will this be tested?

The normal tests should be a lot faster.

[codecov]

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