tests: helpers: teach generate_test_files to return mapped areas

tests: helpers: teach generate_test_files to return mapped areas

This is a preparation to the Fiemap test. In the Fiemap module we have 2
functions: get_mapped_ranges() and get_unmapped_ranges(). And to test both of
them it is convenient to have 2 lists from the
'tests.helpers.generate_test_files()' function: mapped list and unmapped list.

Thus, teach 'tests.helpers.generate_test_files()' to also generate the list of
mapped areas, not only the holes.

Change-Id: Id000ba0b278ccceb821f540755cdd94165eeecc9
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>