test/py: ums: add filesystem-based testing

Enhance the UMS test to optionally mount a partition and read/write a file
to it, validating that the content written and read back are identical.

This enhancement is backwards-compatible; old boardenv contents that don't
define the new configuration data will cause the test code to perform as
before.

test/ums/ is deleted since the Python test now performs the same testing
that it did.

The code is also re-written to make use of the recently added utility
module, and split it up into nested functions so the overall logic of
the test process can be followed more easily without the details
cluttering the code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>