clean up a couple rm calls to better insulate against paths that start with -. while unlikely, the protection is easy

testing/makefiles/libcommon.mk

@@ -47,4 +47,4 @@ ${OBJ_DIR}/%.o: ${top_dir}/testing/%.cpp ${H_FILES}
 		$<
 
 clean:
-	rm -rf "${build_dir}"
+	rm -rf -- "${build_dir}"

testing/makefiles/testcase.mk

@@ -105,7 +105,7 @@ ${OBJ_DIR}/%.o: ${SRC_DIR}/%.cpp
 		$<
 
 clean:
-	rm -f "${SRC_DIR}/generated-testcase.cpp"
-	rm -rf "${build_dir}"
+	rm -f -- "${SRC_DIR}/generated-testcase.cpp"
+	rm -rf -- "${build_dir}"
 
 .PHONY: clean run all build