libdispatch: also build tests during build phase

A tweak to match the Swift CI pattern; build
the test cases during the build phase.

Author: dgrove-oss

utils/build-script-impl

@@ -1795,6 +1795,8 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                 fi
                 pushd "${LIBDISPATCH_BUILD_DIR}"
                 make
+                cd tests
+                make build-tests
                 popd
                 { set +x; } 2>/dev/null