/usr/lib/x86_64-linux-gnu/libXtst.so is in libxtst-dev 2:1.2.2-1+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libXtst.so.6.1.0
/usr/lib/x86_64-linux-gnu/libXtst.so.6.1.0