You presumably have both an iPhone and an iPad - as for iPad to receive SMS/MMS test messaging, this feature must be enabled on the iPhone. If you wish to disable this capability, settings need to be changed on the iPhone.
On your iPhone:
Settings > Messages > Text Message Forwarding - set to OFF - or explicitly disable your iPad from the list of devices.
Also ensure that Messages in iCloud is enabled on your iPad:
Settings > [Your Name / AppleID] > Messages - set to OFF
More information about Apple Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support