[LLDB] dotest.py set selected_platform on remote connection

This patch fixes a bug in dotest.py where lldb.selected_platform was
being set to host platform even after a successful connection to a
remote platform via platform url. This patch fixes this behavior and
sets selected_platform to remote_platform after a successful connection.

This patch also removes target_platform variable from run_suite.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D105060