SPI: test by connecting MOSI to MISO
Usually the SPI Master clocks data out while the Slave clocks data back in. So if you were sending and receiving 8 bits per transaction you can clock out 8 bits and at the same time clock in 8 bits for a total of 8 clocks.  This is described in this Wikipedia paragraph. That said, often the first 8 clocks are used to transmit a command and an additional 8 clocks are used to retrieve the result.

If you looped MOSI to MISO you would clock in what you clocked out. You would complete the entire transaction in 8 clocks. At a low level I'm thinking this does not matter. But at a higher level the software would have to be re-written to accommodate this weirdness.