mmc: mmc_test: add tests to measure random I/O operations per second

Existing performance tests measure single or sequential I/O speed.
Add two random I/O tests:
        33. Random read performance by transfer size
        34. Random write performance by transfer size

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>