FindBoost: search default path if Boost_FIND_VERSION_EXACT
Search paths for boost versions should be build using _boost_TEST_VERSIONS instead of _Boost_KNOWN_VERSIONS because if Boost_FIND_VERSION_EXACT is used _Boost_KNOWN_VERSIONS is empty and boost isn't found even in its default installation path.
Fixes: #17986 (closed)
Topic-rename: FindBoost-exact-version