View HDMI output from another device on a Mid-2015 MacBook Pro
You can use a device such as the Elgato HD60 S, which offers low-latency capture from a 1080p60 HDMI source, which you can view on your MacBook over a USB 3.0 connection. 

It also allows you to save and stream the video if you want to do that as well.

It's available from for example Amazon at $179.95:

https://www.amazon.com/Elgato-Game-Capture-HD60-technology/dp/B01DRWCOGA