Fixed the tests CMake to use current_source_dir instead of just source_dir

This was needed if the tests CMake was added during the root CMake operation.

Also the local include and lib directories as now blocked inside a Win32 guard.
Added the ability to generate the tests solution during the root generation

To accomodate this the TCC_BUILD_TESTS variable is created and checked in the tests CMake.
If it is run in this way the build files will be in root/build/tests/ and the binary files will be in root/bin/[Debug|Release]