security/botan3: Optimize build, add TEST_TARGET

• relax LLVM requirement, per PR #279136, any llvm from ports can compile botan3
• pass --compiler-cache if CCACHE_BIN is nonempty to actually make use of ccache
• replace boost-libs LIB_DEPENDS=libboost_filesystem.so by BUILD_DEPENDS=...asio.hpp to make purpose clearer and avoid the stage-qa warning that libboost_filesystem.so is unused.
• add TEST_TARGET for "make test" and WITH_TESTING[_PORTS].
• strip support for FreeBSD before 1300000, end of life.

PR: 279173
Approved by: fluffy@ (maintainer)