USB wrapper test configuration registers.
| TEST_ENABLE | Enable test of the USB pad. |
| TEST_USB_OE | USB pad oen in test. |
| TEST_TX_DP | USB D+ tx value in test. |
| TEST_TX_DM | USB D- tx value in test. |
| TEST_RX_RCV | USB differential rx value in test. |
| TEST_RX_DP | USB D+ rx value in test. |
| TEST_RX_DM | USB D- rx value in test. |