usb: Fix test failure with multiple partitions

usb: Fix test failure with multiple partitions

When test_usb_load finds multiple partitions of the same type then
it will cause a test failure. The call to write the test file will
write a different test file to each partition but only return the
name and size of the last one written. So the test then fails to
load the test file from the first partition as it uses the name of
a file on a different partition.

Refactor the code so that only one test file is written at a time
and is written to only the partition being tested at that time. This
allows the correct file name to always be available to the code that
runs the load command. This reduces the number of files written and
also the number of calls to crc32 needed.

Fixes: 1c5b6edad381 ("test/py: usb: Add tests for USB device")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Love Kumar <love.kumar@amd.com>