[tests] Fix comment to correct namespace

Author: strongly-typed

test/modm/mock/spi_master.hpp

@@ -128,6 +128,6 @@ class SpiMaster : public modm::SpiMaster
 
 } // namespace platform
 
-} // namespace modm
+} // namespace modm_test
 
 #endif // MODM_TEST_MOCK_SPI_MASTER_HPP