improve UART mock

[2bndy5]

Certain functions/attributes are not present in the mock busio.UART class:

• UART.in_waiting should attempt to read the length of bytes in the next expected operation. Note, this shall not assert that the next expected operation is of type UARTRead.
• UART.reset_input_buffer() should assert that a UARTFlush operation is expected. Note, the UARTFlush operation does not currently exist and should be introduced.
• UART.timeout may be just a dummy attribute. Note, this could be used to check against a UART operation if the UART operations support a dummy wait or timeout attribute. Users should pass a None value to the UARTRead constructor.
• UART.baudrate property should return the value given to the constructor. Note, this should be a read/write property.