Fix bug in a-scene's setAttribute in case of uninitialized systems

[mrxz]

Description:
The setAttribute method in <a-scene> prioritizes systems over components and other attributes. However, it did this by checking if the attribute name was present in the object with initialized systems. When programmatically creating an a-scene these might not have been initialized yet.

This PR changes the condition to first check if the attribute name matches a registered system.

Fixes: #3211

Changes proposed:

• Check if attribute name matches a registered system in setAttribute
• Introduce unit tests for checking <a-scene>'s setAttribute

[dmarcos]

Thanks!