Custom Elements Test Suite

[jridgewell]

We've had 2 cherry-picks related to the new CE v1 polyfill in the last week. We desperately need a Test Suite to verify the correctness of the polyfill.

[dreamofabear]

Should include IE11 coverage.

[samouri]

@jridgewell: can you link to the two bugs and fixes?

I'm guessing it was these two:

1. 🐛 Fix Custom Element instance creation in IE11 #25245
2. 🐛 Upgrade shadow root contents when defining new CEs #25337

[stale]

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