Fix CMake tests handling. BUILD_TESTS + add_subdirectory -> cm_add_test_subdirectory

[x-mass]

Currently entire project uses CMTest not as is has to. All around CMake files you could meet sth like:

if(BUILD_TESTS)
  add_subdirectory(tests)
endif()

But if we read documentation for CMTest, it turns out BUILD_TESTS should be used to either include tests to all target or not.
At the same time, test dirs should be added as cm_add_test_subdirectory(test_subdir) and enabled with CMAKE_ENABLE_TESTS.

Ran into this issue while trying to install crypto3 with make install. Now it installs tests as well, which is not expected. Not sure if this change will affect install target though

[x-mass]

In addition, submodules do not propagate their tests to crypto3, thus we can't run make test.
Will be fixed with BoostCMake/cmake_modules#23

[AndreyMlashkin]

seems to be fixed