mmc_test: collect data and show it via sysfs by demand

Make it possible to get test results via sysfs.  It helps to do tests
non-interactively.  We have the file created under sysfs already and
can use it to show test results.

Prior to this patch, the "test" file under each card's sysfs node was
write-only, and results were obtained by looking at dmesg.  This patch
improves programmatic access to the test results, making them available by
reading back from the same "test" file:

[root@host mmc0:e624]# echo 6 > test
[root@host mmc0:e624]# cat test
Test 6: 2

[[email protected]: changelog improvements]
Signed-off-by: Andy Shevchenko <[email protected]>
Cc: Chris Ball <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Chris Ball <[email protected]>

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