Add build-tests target to build tests separately from make-check
Add build-tests target to build tests separately from make-check

For Swift CI integration it is desirable to have a target
specifically to build the test cases and test utility programs.

frankeh/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,frankeh/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch