trusted-services: copy the libts only once

The libts will be the same for all the trusted services test
applications.  Hence it is not needed to copy it for every test
application.  This will copy the libts only for the last compiled test
application.

Change-Id: Ibcf58ee87b064343ad9604875cbdc13521142e3a
Signed-off-by: Davidson K <davidson.kumaresan@arm.com>