fix: once event is not only firing once

[nicholasgriffintn]

Resolves #393

Description:

The .once event is not actually being fired only once due to the node API in the background being .on rather than .once

Type of change:

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?:

This changes the API that's used in the background for once to also be once

Code changes:

• Fixed the use of the background emitter for once
• Added a test for on and once event emitters