Remove xdescribe from Vector3DTileContentSpec

[ebogo1]

This xdescribe was from this comment in #7249. I ran Travis three times on this branch and all three builds passed; there will be a fourth run for the second commit. If that passes too this should be safe to merge to main.

I don't want to use the "fixes" keyword since #7249 has more than this test failure but that issue can probably be closed after this is merged.

@ggetz Could you please review/merge once CI passes? Thanks!

[cesium-concierge]

Thanks for the pull request @ebogo1!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[mramato]

@ebogo1 did you run these tests locally? Because they still fail for me every time. travis does not run with a real WebGL context, so I'm curious if they pass for anyone else.

[ebogo1]

@mramato I definitely thought I did but I can't get them to pass so maybe not. I'll close the PR since there's more digging to do here.