Open elevated command prompt.
Enter this command.
bcdedit -set TESTSIGNING off
Windows since Windows Vista 64-bit, does not allow unsigned or test-signed kernel mode code (drivers) to be loaded at all.
In order for developers of drivers to be able to properly develop the driver, they need to rapidly load different versions of it into windows. This would require them to get each version signed by Microsoft. That's not reasonable, so Microsoft added the ability to allow loading of test-signed drivers through the use of "Test Mode".
End users don't have much use for this feature and it is a security risk.
The only use for an end user, is if they need to use a driver for some reason that hasn't been certified by Microsoft. Certification costs money, and some independent or amateur developers opt not to do it.
The text cannot be disabled or removed so long as you are in test mode. Only disabling test mode will remove it.