Fix tests on Windows

[traversaro]

• Keep binaries and dll in the same build directory to avoid path issues.
  • Shamelessly copied the strategy from https://github.com/robotology/yarp/blob/aa779bdf3056d90a04acaf8d96dfc09100e960b9/conf/YarpOptions.cmake#L14 .
  • This is also convenient to use a library directly from the build directory.
• Don't use the full path of the test binary in add_test .
  • CMake will automatically expand a target to the full path ( If <command> specifies an executable target (created by add_executable()) it will automatically be replaced by the location of the executable created at build time. https://cmake.org/cmake/help/v3.0/command/add_test.html ).
• Re-enable tests on AppVeyor .

[scpeters]

Nice!