Add tests for --> in the first line

[nicolo-ribaudo]

Tests for tc39/ecma262#3244, which reached consensus today

[nicolo-ribaudo]

@leobalter suggested to add a throw to make sure that the test isn't passing just because everything is considered to be a comment. This is also done in all the other HTML comments tests.

[woess]

These tests should not have flags: [raw] since they're using Test262Error.

I've opened #4020 for this.

[ptomato]

Thanks for opening the issue! I was afraid it would get lost here.