games-engines/box2d: actually run built tests

Not much sense in having a new IUSE=test if not running tests
(testbed still disabled, unsure if can resonably run these here).

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

games-engines/box2d/box2d-2.4.1.ebuild

@@ -25,3 +25,7 @@ src_configure() {
 	)
 	cmake_src_configure
 }
+
+src_test() {
+	"${BUILD_DIR}"/bin/unit_test || die
+}