Test instance beams cannot be retextured independently of beacon beams

The bug

The beam emitted by test instance blocks uses the same file that beacons use. This is inconsistent with end gateways, which use a completely different texture.

When first implemented, end gateways did use the beacon beam texture, but this was changed in the next snapshot or so to make them use a unique texture.

How to reproduce

1. Make a test instance block emit a beam
2. Compare to a beacon
3. Optionally use a resource pack, such as the pack attached to MC-273062, to retexture the beacon beam texture

Expected results

The test beam would have a different resource location from the beacon beam, meaning that it can be retextured differently from the beacon's beam, even if they would appear the same by default.

Actual results

They both point to the same texture file.

How to fix

Copy the beacon beam texture, rename it to something else, and make test blocks use that as the texture instead.