Actually testmy.net is telling you there is an issue somewhere along the testing route. Which is the very reason for it's being.
Running traceroutes can tell us where the lagging router is, checking peer availability , congestion and availability is another.
There was a time where this was not an issue , yet with the recent national routing changes, specific cross continental advertising and video streaming services, not to mention the latest knowledge of the Utah datacenter for national security , and it's special routing procedures , have caused a few peering issues that the ISP cannot and does not have any say in.
For instance , I can test to testmy.net dallas from silicon valley area ( Cupertino ) like it's within my ISP's internal network, but testing from NW Ohio , renders drastically different results. This is not a flaw, this is valuable information.
You can get information on the status of various peers if you so choose here , I use these tools regularly.
Have a look at the image below to understand main routing