Added CXXTEST_LIBPATH to properly use shared libraries

Currently test runners that depend on shared libraries that are outside of standard load path can't be run by SCons cxxtest tool. This makes testing shared libraries impossible  (test runners can't be started due to "can't load shared library" error). 

This patch adds `CXXTEST_LIBPATH` variable. It extends `LIBPATH` used by `Program` builder and `LD_LIBRARY_PATH` (or `PATH` on win32) when running the test.

This patch also makes easier running multiple different test runners in a single project (the `CXXTEST_TARGET` may be given as keyword argument to `CxxTest`).