Add a "rostests_install" target

Currently, the "run_tests" script for the Win2003 WHS-Testbot simply does the following to gather all testing executables:

for exe in `find $directory -name "_.exe" -type f`; do
mv $exe $ROSAUTOTEST_DIR
done

This is a dirty hack and only works for fully self-contained tests.
Nowadays, we also have tests that need external data (see /rostests/testdata) and external DLLs (e.g. localspl_apitest). These tests fail on our Windows 2003 instance at the moment.

The correct way to fix this is adding a "rostests_install" target, which copies all built testing files into a single directory given by an environment variable. The files to copy are named in the CMakeLists.txt file of each test.