mmc: mmc_test: Add tests for sending commands during transfer

Add 6 tests for sending commands during transfer. The tests are:
 * Commands during read - no Set Block Count (CMD23).
 * Commands during write - no Set Block Count (CMD23).
 * Commands during read - use Set Block Count (CMD23).
 * Commands during write - use Set Block Count (CMD23).
 * Commands during non-blocking read - use Set Block Count (CMD23).
 * Commands during non-blocking write - use Set Block Count (CMD23).

For a range of transfer sizes, the tests start an ongoing data transfer and
then repeatedly send the status command (CMD13) while the transfer
continues. The tests pass if all requests complete with no errors. The host
controller driver must support MMC_CAP_CMD_DURING_TFR.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>