macOS Terminal command to test Internet speed
There is a CLI version of Speedtest called, appropriately, speedtest-cli.

You can install it via Homebrew:

brew install speedtest-cli


It's also available via MacPorts

Just execute it by running the command:

$ speedtest-cli


Retrieving speedtest.net configuration...
Testing from ACME Cable (xxx.xxx.xxx.xxx)...
Retrieving speedtest.net server list...
Selecting best server based on ping...
Hosted by ServerPronto (Miami, FL) [22.39 km]: 25.419 ms 
Testing download speed............................................................
Download: 70.16 Mbit/s
Testing upload speed............................................................
.Upload: 5.92 Mbit/s