How can I find and install libXtst.so.6 in Centos 7 for scenebuilder-8.3.0-1.x86_64

Instead of downloading the RPM, you can try installing it from the CentOS repositories with the following (as root):

yum install libXtst

That should pull in any other dependencies and update any packages that require that.

If the 64-bit package is already installed then you may need to install the 32-bit library

yum install libXtst.i686