Fix for xt-mouse-tests.el broken in prior commit

Fix for xt-mouse-tests.el broken in prior commit

xt-mouse-tests not turning on xterm-mouse-mode if it was already
on did not work with xterm-mouse-mode being set to t by default.
It turns out that xt-mouse-tests already does not report proper
results when called inside a running Emacs session so the code
path is not useful anyways.

* test/lisp/xt-mouse-tests.el (with-xterm-mouse-mode): Always
turn on xterm-mouse-mode, even if it is already on.

(cherry picked from commit 2719dccbe5797dd8fcf1ce013df2ff71161b6f55)